D is the correct answer.

In most biochemical pathways, only a few enzymatic reactions are under regulatory control. These often occur either at the beginning of pathways or at pathway branch points. The pyruvate dehydrogenase (PDH) complex controls the link between glycolysis and the citric acid cycle, and decarboxylates pyruvate (the end product of glycolysis) with production of NADH and acetyl-CoA (the substrate for the citric acid cycle). After intense exercise, one would expect PDH to be highly active to generate ATP. ADP levels (choice (A)) should be high because ATP was just burned by the muscle. Acetyl-CoA (choice (B)) is an inhibitor of PDH, causing a shift of pyruvate into the gluconeogenesis pathway. A high NADH/NAD⁺ ratio (choice (C)) would imply that the cell is already energetically satisfied and not in need of energy, which would not be expected in intensely exercising muscle.