Krebs cycle occurs in the matrix of the mitochondrion and uses oxygen.

• A. Glycolysis
• B. Pyruvate oxidation
• C. Krebs cycle
• D. Electron transport chain

Answer: (C)

The key idea is the location of the process and how oxygen supports the whole aerobic pathway. The Krebs cycle runs in the mitochondrial matrix. It does not directly use oxygen as a substrate, but its continuation depends on reoxidizing NADH and FADH2 through the electron transport chain, which requires oxygen as the final electron acceptor. Without oxygen, NAD+ and FAD would not be regenerated, and the cycle would stall. Among the options, glycolysis happens in the cytoplasm, pyruvate oxidation occurs in the matrix but doesn’t directly use O2, and the electron transport chain operates in the inner membrane and uses O2 there. So the Krebs cycle is the best match for both the location and the aerobic context.