The citric acid cycle enzyme that catalyzes the reaction directly responsible for the production of succinyl-CoA is alpha-ketoglutarate dehydrogenase. Alpha-ketoglutarate dehydrogenase catalyzes the conversion of alpha-ketoglutarate, , and CoA-SH to succinyl-CoA, NADH, , and . 

Each of the other enzymes listed are enzymes that participate in the citric acid cycle, but not in the step the directly produces succinyl-CoA. Their general roles are as follows:

Succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA to succinate.

Succinate dehydrogenase catalyzes the conversion of succinate to fumarate.

Isocitrate dehydrogenase catalyzes the conversion of isocitrate to alpha-ketoglutarate.

Fumarase catalyzes the conversion of fumarate to malate.

The only correct statement within the answer choices is that succinyl-CoA synthetase catalyzes the reaction responsible for the formation of succinate. In this reaction, the citric acid cycle enzyme, succinyl-CoA synthetase, catalyzes the conversion of succinyl-CoA, GDP (or ADP) and inorganic phosphate to succinate, CoA-SH, and GTP (or ATP). 

The incorrect answer choices are explained below:

"Succinyl-CoA synthetase is produced by the reaction catalyzed by alpha-ketoglutarate dehydrogenase."

This is incorrect because it is succinyl-CoA that is produced by the reaction catalyzed by alpha-ketoglutarate dehydrogenase. Succinyl-CoA synthetase, the enzyme, is not produced by this reaction.

"Succinyl-CoA synthetase catalyzes the reaction responsible for the rate-limiting step of glycolysis."

Succinyl-CoA synthetase does not participate in glycolysis; it participates in the citric acid cycle. Furthermore, even in the citric acid cycle, it does not catalyze the reaction responsible for the rate-limiting step of the citric acid cycle.

"Succinyl-CoA synthetase catalyzes the reaction responsible for the formation of succinyl-CoA."

Alpha-ketoglutarate dehydrogenase catalyzes the reaction responsible for the formation of succinyl-CoA. Succinyl-CoA synthetase catalyzes the reaction responsible for the conversion of succinyl-CoA to succinate.

"Succinyl-CoA synthetase directly catalyzes a citric acid cycle reaction that produces ."

Succinate dehydrogenase directly catalyzes a citric acid cycle reaction that produces , not succinyl-CoA synthetase.

The rate-limiting step of the citric acid cycle is catalyzed by the enzyme, isocitrate dehydrogenase. Isocitrate dehydrogenase catalyzes the conversion of isocitrate and  to alpha-ketoglutarate, NADH, a proton, and a molecule of carbon dioxide. 

Phosphofructokinase-1 (PFK-1) is incorrect, as it catalyzes the rate-limiting step of glycolysis, not the citric acid cycle.

Fructose 1,6-bisphosphatase is incorrect, as it catalyzes the rate-limiting step of gluconeogenesis, not the citric acid cycle.

Succinyl-CoA synthetase is incorrect, as it catalyzes a reaction within the citric acid cycle that is not the rate-limiting step.

Pyruvate kinase is incorrect as the reaction that it catalyzes is neither within the citric acid cycle, nor a rate-limiting step.

Phosphoenolpyruvate (PEP) is an intermediate in glycolysis, not the citric acid cycle. PEP is the product of the ninth reaction in glycolysis, which involves the enolase-catalyzed conversion of 2-phosphoglycerate into PEP. All other molecules are indeed intermediates in the citric acid cycle.

The three-carbon molecule pyruvate produced from glycolysis 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; this third carbon from pyruvate was lost as carbon dioxide during its conversion to acetyl-CoA via the pyruvate dehydrogenase complex.

The citric acid cycle begins when a four-carbon molecule, oxaloacetate combines with acetyl-CoA (a two carbon molecule) to produce the six-carbon molecule citrate. The enzyme citrate synthase carries out this reaction. Citrate then becomes the six-carbon molecule cis-aconitate via catalysis by aconitase. The same enzyme then converts cis-aconitate to isocitrate, which is an isomer of citrate.

Thiamine (B1) acts as a cofactor to enable pyruvate dehydrogenase to convert pyruvate from glycolysis into acetyl-CoA so it can enter the citric acid cycle. 

Oxaloacetate combines with acetyl-CoA to form citrate. This is the first stage of the citric acid cycle. Eventually, citrate will lose two molecules of  to regenerate the four-carbon molecule oxaloacetate.

During strenuous exercise, GLUT4 will be active to bring glucose into the cell. Glucose is then pushed through glycolysis to generate pyruvate. Pyruvate is then pushed through pyruvate dehydrogenase complex, where it is converted into acetyl-CoA, which feeds into the Krebs cycle (assuming aerobic conditions) to generate ATP, NADH, , and carbon dioxide.  

Most of the intermediate molecules in the Krebs cycle can, rather than continuing through the cycle itself, go through other pathways to form macromolecules. Citrate can be used to create fatty acids and sterols. Alph-ketoglutarate can be used to make some of the amino acids. Succinyl-CoA can be used to make porphyrins, heme, and chlorophyll. Aspartate can be used to make some of the amino acids. Oxaloacetate can be used in gluconeogenesis to create glucose.