This question is providing us with a scenario in which  ions enter a cell. We're further told that it will take a single molecule of ATP to move three of these ions out of the cell. Finally, we are being asked to determine the total number of acetyl-CoA molecules that must pass through the Krebs cycle in order to provide the energy necessary for the export of these  ions.

First, we'll need to determine the total number of ATP molecules generated from the passage of a single molecule of acetyl-CoA through the Krebs cycle. It's important to remember that the passage of acetyl-CoA through the Krebs cycle generates one molecule of ATP directly by substrate-level phosphorylation, but it also produces other intermediate energy carriers in the form of  and .

For each acetyl-CoA ran through the cycle, one molecule of  and three molecules of  are produced. Furthermore, each molecule of  will go on to donate its electrons to the electron transport chain to generate  molecules of ATP per molecule of  oxidized. Likewise, each  will also produce ATP via oxidative phosphorylation, but at a rate of  molecules of ATP per molecule of  oxidized.

Adding these up, we obtain:

 ATP via substrate-level phosphorylation

Adding these values up, we have a total of  molecules of ATP produced for every molecule of acetyl-CoA oxidized. Now that we know how much ATP is produced from one acetyl-CoA, we can calculate the number needed to move the  ions out of the cell.

The only step of the citric acid cycle (also known as the Krebs cycle, or the TCA cycle) that directly produces ATP or GTP is the conversion of succinyl-CoA to succinate. 

In this reaction, succinyl-CoA is converted to succinate with the assistance of the enzyme, succinyl-CoA synthetase. During this reaction, ADP + Pi (or GDP + Pi) is also converted to ATP (or GTP) using the energy from the breaking of the bond between CoA and succinate. Thus, the overall reaction appears as:

While side products of some of the other reactions listed produce intermediaries that may be used to produce ATP in the future, these reactions do not directly produce ATP.

The only citric acid cycle (also known as the Krebs cycle or TCA cycle) step listed that does not result in the production of  as a side product is the conversion of fumarate to malate. 

In the conversion of fumarate to malate, fumarate is chemically combined with water in the presence of the enzyme fumarase to produce malate. In this conversion, there is no concomitant production of .

In each of the other reactions listed,  is converted to  and  as side products.  

The correct answer is that none of the citric acid cycle steps listed result in the production of . The only step of the citric acid cycle that results in the production of  is the conversion of succinate to fumarate (catalyzed by succinate dehydrogenase). In this reaction,  is concomitantly converted to  using the hydrogen molecules removed from succinate by succinate dehydrogenase. This reaction was not listed in the answer choices though, and therefore none of the reactions listed produced .

Each of the reactions listed did produce other side products. The conversions of isocitrate to alpha-ketoglutarate, alpha-ketoglutarate to succinyl-CoA, and malate to oxaloacetate all result in the production of  as a side product, but not . The conversion of succinyl-CoA to succinate results in the production of ATP or GTP and CoA-SH as side products, but not . 

Flavin mononucleotide (FMN) is not produced by the citric acid cycle. This flavin coenzyme is a reactant, but not a product, since FMN will get reduced to FMNH₂.

The rest of the answer choices are products of the citric acid cycle (otherwise known as the Krebs cycle). 

Pyruvate is the end product of glycolysis. After glycolysis, the three-carbon molecule pyruvate is converted into the two-carbon molecule acetyl-coenzyme A (acetyl-CoA). This is carried out by a combination of three enzymes collectively known as the pyruvate dehydrogenase complex. The conversion of pyruvate to acetyl-CoA also produces one molecule of . Acetyl-CoA has one less carbon than pyruvate. The third carbon from pyruvate is lost as carbon dioxide () during the conversion of pyruvate to acetyl-CoA. Recall that since glucose is a six-carbon molecule, two molecules of pyruvate (three carbons each) are formed via glycolysis.

Pyruvate is produced in the last step of glycolysis, then, it is converted to the two-carbon molecule acetyl-coenzyme A (acetyl-CoA). This is carried out by a combination of three enzymes collectively known as the pyruvate dehydrogenase complex. The conversion of pyruvate to acetyl-CoA produces one . Acetyl-CoA has one less carbon than pyruvate. The third carbon of pyruvate is lost as carbon dioxide () during the conversion of pyruvate to acetyl-CoA. The citric acid cycle begins when the four-carbon molecule, oxaloacetate combines with acetyl-CoA (a two carbon molecule) via an aldol condensation, yielding the six-carbon molecule citrate.

During the conversion of succinate into fumarate by succinate dehydrogenase, a single molecule of  is reduced to  as it accepts the hydrogens from succinate.  then feeds its electrons into the electron transport chain in the inner mitochondrial membrane.

Succinyl-CoA synthetase performs substrate level phosphorylation at this step in the Krebs cycle, such that .

The conversion of pyruvate to acetyl-CoA produces one molecule of NADH, but remember that each glucose yields two pyruvates, so the total NADH from this first step is two. Within the citric acid cycle, there are three steps in which NADH is a byproduct, but again we must remember that each step occurs to two molecules, therefore three NADH byproducts for two molecules yields six NADH in the cycle proper. Therefore, the total NADH produced in one turn of the citric acid cycle is eight NADH.