d. it diffuses into mitochondria to be broken down to generate ATP
Explanation:
When enough oxygen is available in the muscle cells, pyruvate produced by glycolysis enters the mitochondrial matrix. Once inside the mitochondria, pyruvate is decarboxylated into acetyl CoA. The reaction is catalyzed by the enzyme complex pyruvate dehydrogenase. Acetyl CoA then enters a sequence of reactions called Kreb's cycle and is broken down into CO2 and H2O. The energy released during these reactions is stored in the form of NADH and FADH2.
The NADH and FADH2 are oxidized by giving their electrons to O2 via electron transport chain. During this oxidation, the proton concentration gradient is generated across the inner mitochondrial membrane which in turn drives the process of ATP synthesis.
