Citric acid cycle inputs are derived from glycolysis outputs. Glycolysis produces pyruvate molecules, \(\displaystyle NADH\), and ATP. The pyruvate molecules undergo reactions that convert the three carbon pyruvate to a two carbon acetyl CoA and an one carbon carbon dioxide. The acetyl-CoA molecules are then used as the initial inputs for the citric acid cycle, as they are combined with oxaloacetate. Note that pyruvate itself does not enter the citric acid cycle. \(\displaystyle FADH_2\) and \(\displaystyle NADH\)are electron carriers that are produced in the citric acid cycle and are used in electron transport chain to generate ATP.

A glucose (six carbons) molecule enters glycolysis and produces two three carbon molecules (pyruvate). Each pyruvate is broken down into a two carbon acetyl-CoA molecule that enters the citric acid cycle. Each acetyl-CoA molecule produces three \(\displaystyle NADH\) and one \(\displaystyle FADH_2\) in the citric acid cycle. This means that two acetyl-CoA (derived from one glucose molecule) produces six \(\displaystyle NADH\) and two \(\displaystyle FADH_2\) molecules in the citric acid cycle.

Citric acid cycle involves a series of reactions that are involved in the production of the necessary molecules for electron transport chain. The cycle starts with a two carbon molecule (acetyl-CoA) binding to a four carbon molecule (oxaloacetate). This creates a six carbon molecule (citrate) that can go through a series of reactions. Most of these reactions involve a six carbon molecule. As mentioned, acetyl-CoA has two carbons; therefore, most of the intermediates in this cycle have six carbons, or four more carbons than acetyl-CoA. One turn of citric acid cycle produces \(\displaystyle 3 \: NADH\), \(\displaystyle 1\:FADH_2\), \(\displaystyle 2\: CO_2\) (carbon dioxide) and one GTP molecule(s).

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, \(\displaystyle NAD^+\) is reduced to \(\displaystyle NADH\).

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 \(\displaystyle 6 \ NADH\) and \(\displaystyle 2\ FADH_2\).

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 \(\displaystyle CO_2\). In the mitochondria, the reaction produces also a charged electron carrier molecule, \(\displaystyle NADH\), from \(\displaystyle NAD^+\). Isocitrate dehydrogenase, inhibited by \(\displaystyle NADH\) and activated by \(\displaystyle ADP\), is a major regulator enzyme of the citric cycle.

The only step of the citric acid cycle listed that results in the production of \(\displaystyle CO_2\) 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 \(\displaystyle NAD^+\) to \(\displaystyle NADH\) and \(\displaystyle H^+\) as side products, and generating a molecule of \(\displaystyle CO_2\) 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 \(\displaystyle CO_2\) as a side product. However, this step is not listed as an answer choice.

None of the other answer choices listed produce \(\displaystyle CO_2\) as side products.