Final answer:
The model that represents the intermediate step between glycolysis and the citric acid cycle is acetyl-CoA (Option D).
Step-by-step explanation:
Acetyl-CoA is the intermediate step between glycolysis and the citric acid cycle. It is formed from pyruvate by the enzyme pyruvate dehydrogenase within the mitochondrion before entering the citric acid cycle. After glycolysis produces two molecules of pyruvate, the pyruvate undergoes a transition by the enzyme pyruvate dehydrogenase into acetyl-CoA. This process is known as an oxidative decarboxylation reaction, where the three-carbon pyruvate is converted to a two-carbon acetyl-CoA molecule, releasing carbon dioxide and capturing high-energy electrons in NADH.
The acetyl-CoA then enters the citric acid cycle within the matrix of the mitochondrion, beginning the series of reactions that will harvest the remainder of the energy from the original glucose molecule.
Thus, the correct option is D.