The first step in the citric acid cycle is for acetyl-CoA to react with oxaloacetate.  This forms citrate, which then continues through the cycle, ultimately reforming the oxaloacetate molecule to redo the cycle.

Pyruvate is converted to acetyl-CoA by the pyruvate dehydrogenase complex. Carbon dioxide is released during this reaction, and in addition to this,  is reduced to .

The pyruvate dehydrogenase complex (PDC) is preparing pyruvate for the Krebs cycle by converting it to acetyl-CoA. Because the Krebs cycle functions within the mitochondrial matrix, the PDC is also taking place there.  This ensures quick and easy movement from the PDC into the Krebs cycle.

The Krebs cycle produces the most high energy electron carriers of any process involved in cellular respiration. Per glucose molecule, the Krebs cycle produces  and .

Ubiquinone is a part of the electron transport chain, and has little to do with the Krebs cycle. Glucose is broken down during glycolysis, and does not enter the Krebs cycle directly. Many students make the mistake of thinking that pyruvate enters the Krebs cycle, since it is produced in glycolysis, and the Krebs cycle follows glycolysis. However, pyruvate is first converted to acetyl-CoA by the pyruvate dehydrogenase complex in the mitochondrial matrix, and acetyl-CoA enters the Krebs cycle.

Isocitrate dehydrogenase activation leads to oxidative decarboxylation of isocitrate in a two step process producing alpha-ketoglutarate and . In the mitochondria, the reaction produces also a charged electron carrier molecule, , from . Isocitrate dehydrogenase, inhibited by  and activated by , is a major regulator enzyme of the citric cycle.

The only step of the citric acid cycle listed that results in the production of  as a side product is the conversion of isocitrate to alpha-ketoglutarate. In this step, the enzyme, isocitrate dehydrogenase catalyzes the conversion of isocitrate to alpha-ketoglutarate, while also converting  to  and  as side products, and generating a molecule of  in the process (i.e. reducing the carbon count from 5 in isocitrate to 4 in alpha-ketoglutarate). 

The conversion of alpha-ketoglutarate to succinyl-CoA also produces a molecule of  as a side product. However, this step is not listed as an answer choice.

None of the other answer choices listed produce  as side products. 

The purpose of the citric acid cycle is to produce energy (both directly via GTP, and indirectly via NADH and . As such, energy can be though of to be on the products side of the sum of the reactions of the Krebs cycle. From Le Chatelier's principle, we know that if we want to inhibit a forward reaction, we can increase the concentration of the products. This will inhibit the forward reaction, and push the equilibrium to the left. Thus, in a high energy state, the ratio of ATP:ADP, like that of NADH: is high since both ATP and NADH are products of metabolism.

The entry point for acetyl CoA in the Krebs cycle is oxaloacetate. Acetyl-CoA and oxaloacetate combine to form citrate, which then continues through the cycle. Oxaloacetate is a carbohydrate, and so without carbs the acetyl-CoA can not enter into the Krebs cycle.

The regulated steps of the citric acid cycle are citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase. These steps are inhibited and stimulated by various products and reactants within the citric acid cycle. Succinate dehydrogenase is not regulated by products or reactants, and is therefore not rate limiting.