Without a terminal electron acceptor, the electrons of  and  would have nowhere to be released, all of the complexes would be "backed up" as each complex would not be able to pass off its electrons to the next complex. ATP production would come to a standstill.

Without oxygen to receive the electrons, the entire flow of the electron transportation chain halts, as well as ATP production. It is the continuous flow of electrons through the ETC complexes that allows a mitochondria to harness the energy of the electrons that  and  donate. This energy is used to pump protons across the intermembrane space of a mitochondria. The re-entry of these protons through ATP synthase is what drives the production of ATP.

In short, no electron flow means no proton pumps and no re-entry of those protons through ATP synthase. A cell could potentially resort to glycolysis to produce ATP, and can regenerate  or  using anaerobic fermentation such as alcohol fermentation of lactic acid fermentation.

The electron transport chain passes electrons thru its main components: complex I (NADH dehydrogenase), coenzyme Q, complex III, cytochrome C, and complex IV. Complex IV is the cytochrome oxidase complex and it is inhibited by cyanide, carbon monoxide and azide. Cyanide binds irreversibly to complex IV preventing electron transfer.

Reactive oxygen species are superoxide, hydrogen peroxide, and hydrogen radicals. They are degraded by catalase, superoxide dismutase, and glutathione peroxidase. Neutrophils use reactive oxygen species to kill bacteria during the phagocytic oxidative burst. 

Uncouplers of the electron transport chain decrease the proton gradient and thus decrease ATP synthesis. Most energy from the electron transport chain is released as heat. The most common uncouplers are 2,4-dinitrophenol and aspirin, as well as thermogenin. Carbon monoxide is an inhibitor of the electron transport chain, not an uncoupler. Nitric oxide does not affect directly the electron transport chain.

Oxygen is an electron acceptor. In the absence of oxygen (hypoxia) cells cannot generate ATP in the mitochondria. Instead, they will utilize glycolysis. Oxygen is required to carry out the electron transport chain and produce ATP via oxidative phosphorylation.

The electron transport system is the only metabolic process listed that directly requires molecular oxygen. Oxygen is the final electron acceptor (it is one of the most electronegative atoms in our bodies) in the electron transport chain. This is the same as saying that oxygen has the highest reduction potential, and is capable of receiving electons. If oxygen is not present to accept the electron from the final enzyme complex in the inner mitochondrial membrane, then electron transport will be inhibited and thus no ATP will be produced via chemiosmosis.

Note that the Krebs cycle, citric acid cycle, and tricarboxylic acid cycle (TCA cycle) all refer to the same process, and do not directly require oxygen (oxygen is neither a reactant nor a product in any of the steps). However, oxygen is indirectly required, as there is no point to this cycle without subsequent oxidative phosphorylation. Thus in the absence of oxygen, of the choices shown, only glycolysis will proceed uninhibited.

The electron transport chain generates the most ATP out of all three major phases of cellular respiration. Glycolysis produces a net of 2 ATP per molecule of glucose. In the Krebs cycle, there is one GTP (which is an ATP equivalent) generate in the conversion of succinyl-CoA to succinate. However, the majority of the ATP produced during cellular respiration occurs at the electron transport chain by the reduction of coenzymes NADH and . This subsequently results in the generation of the proton motive force which ATP synthase uses to generate ATP from one unit of ADP and one unit of inorganic phosphate.

In a eukaryote, oxidative phosphorylation occurs in the mitochondria because this is where the cell is able to set up a proton gradient.  However, prokaryotes do not have mitochondria - they have no membrane-bound organelles at all.  Therefore, the proton gradient that drives ATP synthesis in oxidative phosphorylation is created across the cell membrane.

With the excess buildup of protons in the matrix, the only thing that will be inhibited is the generation of ATP by ATP synthase. The other processes in cellular respiration focus more on creation of high energy electron carriers, and therefore will continue as normal.

Predictably, a gain in oxygen is known as oxidation, while a loss of oxygen is reduction.  Hydrogen follows the opposite pattern as oxidation: removing hydrogen is oxidation, while gaining hydrogen is reduction.  Therefore, the correct answer is that removing hydrogens is considered oxidation.

In order to differentiate between oxidation and reduction in terms of electron transfer, it is helpful to remember the phrase "LEO the tiger says GER".  A loss of electrons is oxidation, while a gain of electrons is reduction.