A positive Gibbs free energy implies that the process in question should be unfavorable under normal conditions.  The only process listed that is unfavorable and requires an input of energy is the pumping of hydrogen ions into the intermembrane space.  This occurs during the electron transport chain.

The phases of cellular respiration are glycolysis, pyruvate dehydrogenase complex, Krebs cycle, electron transport chain. Glycolysis produces a net total of 2 ATP, the Krebs cycle produces 1 GTP that is converted to ATP in another process, and the electron transport chain is where almost all of the ATP made in cellular respiration is formed. However, during the pyruvate dehydrogenase complex phase of cellular respiration, pyruvate is converted to acetyl-CoA as a preparation for the Krebs cycle, but no ATP is created.

The correct answer is cyanide. This compound acts to inhibit cytochrome C oxidase, otherwise known as Complex IV of the electron transport chain. By inhibiting this complex, cyanide effectively halts the flow of electrons through the chain. Consequently, protons are not able to be pumped from the matrix to the intermembrane space and thus, a proton gradient cannot be established. Without a proton gradient, protons will not flow through ATP synthase, hence no ATP will be produced.

Oligomycin, on the other hand, acts to inhibit ATP synthase, which means that ATP will not be able to be produced. However, electrons are still able to flow through the chain, which means that protons are still able to be pumped across the inner membrane.

Dinitrophenol is a relatively nonpolar compound that is able to situate itself into the inner mitochondrial membrane. In doing so, it is able to dissipate the proton gradient by allowing protons to essentially be transported from the intermembrane space to the matrix without traversing through ATP synthase. Even though protons can still flow through ATP synthase to generate ATP in this scenario, the proton gradient won't be nearly as potent because they now have an alternative route to the matrix.

Methotrexate acts to competitively inhibit the enzyme known as dihydrofolate reductase. This enzyme has nothing to do with the electron transport chain, and thus will have no effect on ATP synthesis. Commonly, this drug is used as an anti-cancer agent because its substrate, dihydrofolate, is a compound that is used in the syntheis of thymine nucleotides for DNA synthesis. Inhibiting dihydrofolate reductase effectively reduces the production of thymine, which can negatively impact DNA replication in rapidly dividing cancer cells.

Likewise, allopurinol has nothing to do with the electron transport chain. This drug acts as an inhibitor of the enzyme xanthine oxidase, which is responsible for synthesizing uric acid. High levels of uric acid can lead to the development of gout, and thus, this drug is typically used to help treat people suffering from gout.

Potassium cyanide inhibits cellular respiration by acting on mitochondrial cytochrome c reductase (leading to hypoxia and death). Rotenone also affects oxidative phosphorylation, by inhibiting electron transfer from cytochrome 1 to ubiquinone, making it a potent insecticide. Oligomycin inhibits ATP synthase, also slowing flow of the electron transport chain. Fructose 2,6-biphosphate affects the activity of enzymes regulating glycolysis and gluconeogenesis. Dinitrophenol dissipates the proton gradient across mitochondrial membranes, and shuttles protons across them, inhibiting ATP production. 

Aerobic respiration is a process that utilizes the electron transport chain in order to oxidize glucose into energy. If a chemical were added that inhibited the electron transport chain, the cell would no longer be able to fully oxidize glucose. Therefore, oxygen consumption will decrease. Furthermore, since the cell is now in a situation in which it is not able to make as much energy per glucose molecule as before, it will need to increase its consumption of glucose in order to generate enough energy through anaerobic respiration alone.

Oligomycin is an antibiotic that inhibits ATP synthase. It works by binding to the stalk of ATP synthase. This prevents proton re-entry into the mitochondrial matrix. This results in a halt of the proton motive force that ATP synthase uses to created ATP from one unit of ADP and one unit of inorganic phosphate. Rotenone is a pesticide and fish poison that inhibits NADH dehydrogenase in complex I causing the levels of NADH to increase. This results in a halt of the electron transport chain. Antimycin is a fungal antibiotic that inhibits complex III of the electron transport chain. Antimycin prevents the transfer of electrons through the cytochrome b-c complex. Cyanide is a gas that inhibits complex IV of the electron transport chain. Cyanide combines with cytochrome oxidase and prevents the transfer of electrons to oxygen.

Antimycin is a fungal antibiotic that inhibits complex III of the electron transport chain. Antimycin prevents the transfer of electrons through the cytochrome b-c complex. Oligomycin is an antibiotic that inhibits ATP synthase. It works by binding to the stalk of ATP synthase. This prevents proton re-entry into the inner mitochondrial matrix. This results in a halt of the proton motive force that ATP synthase uses to created ATP from one unit of ADP and one unit of inorganic phosphate. Rotenone is a pesticide and fish poison that inhibits NADH dehydrogenase in complex I causing the levels of NADH to increase. This results in a halt of the electron transport chain. Cyanide is a gas that inhibits complex IV of the electron transport chain. Cyanide combines with cytochrome oxidase and prevents the transfer of electrons to oxygen.

Rotenone is a pesticide and fish poison that inhibits NADH dehydrogenase in complex I causing the levels of NADH to increase. This results in a halt of the electron transport chain. Oligomycin is an antibiotic that inhibits ATP synthase. It works by binding to the stalk of ATP synthase. This prevents proton re-entry into the mitochondrial matrix. This results in a halt of the proton motive force that ATP synthase uses to created ATP from one unit of ADP and one unit of inorganic phosphate. Antimycin is a fungal antibiotic that inhibits complex III of the electron transport chain. Antimycin prevents the transfer of electrons through the cytochrome b-c complex. Cyanide is a gas that inhibits complex IV of the electron transport chain. Cyanide combines with cytochrome oxidase and prevents the transfer of electrons to oxygen.

Cyanide is a gas that inhibits complex IV of the electron transport chain. Cyanide combines with cytochrome oxidase and prevents the transfer of electrons to oxygen. Antimycin is a fungal antibiotic that inhibits complex III of the electron transport chain. Antimycin prevents the transfer of electrons through the cytochrome b-c complex. Oligomycin is an antibiotic that inhibits ATP synthase. It works by binding to the stalk of ATP synthase. This prevents proton re-entry into the mitochondrial matrix. This results in a halt of the proton motive force that ATP synthase uses to created ATP from one unit of ADP and one unit of inorganic phosphate. Rotenone is a pesticide and fish poison that inhibits NADH dehydrogenase in complex I causing the levels of NADH to increase. This results in a halt of the electron transport chain.

CoQ₁₀ is produced in the liver and oxidizes enzyme complex II. It is subsequently oxidized by enzyme complex III in the ETC. Oxygen is the final acceptor of electrons in the ETC.