During oxidative phosphorylation,  is created from the previously created  and . All of the other choices describe other parts of cellular respiration. In glycolysis, glucose is oxidized to pyruvate. In both glycolysis and the Krebs cycle, substrate level phosphorylation occurs. Likewise,  and  are produced during glycolysis and the Krebs cycle, but not during oxidative phosphorylation, where these high energy electrons are passed down a series of membrane-bound enzymes to oxygen meanwhile protons are pumped into the intermembrane space of the mitochondria.

To answer this question, it's important to have familiarity with the process of aerobic respiration.

In the first major pathway, glycolysis is split into two molecules of pyruvate through a series of reactions. Along the way, high-energy electron carriers are produced, along with ATP.

In the next major step, pyruvate is transferred into mitochondria, where it is decarboxylated into acetyl-CoA, with a concomitant production of NADH and carbon dioxide. Hence, this is a step that produces carbon dioxide. However, it is not found in the answer choices.

The third major component of aerobic respiration is the citric acid cycle. Here, the acetyl-CoA from the previous step is completely ripped apart to provide a great deal of energy. This huge amount of energy that is liberated is because the two carbon atoms that make up the acetyl group of acetyl-CoA become completely oxidized into two molecules of carbon dioxide. In terms of the energy liberated from the cycle, ATP along with a good deal of high-energy electron carriers are produced. This component of aerobic respiration is indeed a source of carbon dioxide.

Fermentation is an anaerobic pathway and is thus not the correct answer. Depending on the organism, carbon dioxide may or may not be produced.

Finally, aerobic respiration culminates in oxidative phosphorylation. Here, all of the high energy carriers from the previous steps are fed into the electron transport chain, resulting in the production of a great amount of ATP, the main energy currency of cells. In this final major step, it is oxygen gas that is produced, not carbon dioxide.

Glycolysis occurs in the cytoplasm. Pyruvate dehydrogenase complex occurs in the mitochondrial matrix. Krebs cycle occurs in the mitochondrial matrix. Electron transport chain protein complexes are embedded in the mitochondrial inner membrane.

Aconitase is the enzyme that catalyzes the conversion of citrate to isocitrate. This essential enzyme is vital in energy production, as it acts like an iron regulatory protein. The conversion of citrate to isocitrate is important since it is needed to react with isocitrate dehydrogenase.

Citrate synthase is the first enzyme of the citric acid cycle. Its role is to condense acetyl-CoA onto oxaloacetate in order to generate citrate.

Acetyl-CoA carboxylase is an enzyme that attaches a carboxyl group to acetyl-CoA in order to generate malonyl-CoA, which plays a role in fatty acid synthesis by contributing two carbons at a time to the growing hydrocarbon chain. Moreover, malonyl-CoA also serves a regulatory role in the breakdown and synthesis of fatty acids. Since it is a major precursor to the synthesis of fatty acids, high levels of it inhibit the breakdown of fatty acids by preventing fatty acids from entering the mitochondria, where they are broken down via beta-oxidation. Thus, malonyl-CoA allows fatty acids to be synethesized without simultaneously being degraded.

Thiolase is an enzyme that condenses two molecules of acetyl-CoA into acetoacetyl-CoA. This molecule is an important intermediate in two important pathways. One is the production of ketone bodies, while the other is the mevalonate pathway, which is an important series of reactions that synthesizes many compounds, such as cholesterol.

Pyruvate dehydrogenase is an enzyme complex that converts pyruvate into acetyl-CoA, thus linking glycolysis with the citric acid cycle.

Pyruvate carboxylase is an enzyme that adds a carboxyl group to pyruvate in order to generate oxaloacetate. This reaction can be used either to generate oxaloacetate for use in the kreb's cycle, or in the gluconeogenesis pathway to synthesize glucose from a variety of substrates.

The first step of the citric acid cycle involves the combination of acetyl-CoA with oxaloacetate, producing citrate. Next, aconitase catalyzes the isomerization of citrate to isocitrate, via the intermediate known as cis-aconitate. The conversion of isocitrate to alpha-ketoglutarate is catalyzed by isocitrate dehydrogenase. The citric acid cycle involves the conversion of fumarate to malate, not the reverse, although some bacteria do the reverse citric acid cycle to produce complex organic compounds. Succinyl-CoA is converted to succinate via succinyl-CoA synthetase (also known as succinate thiokinase).

Succinate dehydrogenase (also known as succinate-coenzyme Q reductase or complex II) is bound to the inner mitochondrial membrane. It participates in both the citric acid cycle and the electron transport chain. Aconitase is an enzyme involved in glycolysis, not the citric acid cycle (Krebs cycle).

All enzymes that have NADH as a product would be inhibited by the addition of NADH. The correct answers are citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase. Pyruvate dehydrogenase, which synthesizes acetyl-CoA from pyruvate, is also inhibited by NADH.

Each enzyme listed in the answer choices catalyzes a reaction in glycolysis or the citric acid cycle, but pyruvate dehydrogenase is the only one responsible for converting pyruvate to acetyl-CoA. Pyruvate is decarboxylated (removal of a ) and this is replaced with an S-CoA group, forming the necessary acetyl-CoA molecule to feed into the citric acid cycle. 

Succinate dehydrogenase is the only enzyme in the citric acid cycle to use . The other dehydrogenases use  while citrate synthase performs an unrelated reaction using acetyl-CoA.