Final answer:
The electron transport chain is the step in cellular respiration that produces the most ATP as it is the most efficient in ATP production compared to glycolysis, pyruvate oxidation, and the Krebs cycle.
Step-by-step explanation:
The step of cellular respiration that produces the most ATP is the electron transport chain (C). During cellular respiration, glucose is broken down in several steps: glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation, which includes the electron transport chain and chemiosmosis. Among these, the electron transport chain is the most efficient in ATP production. Glycolysis converts glucose to pyruvate, pyruvate oxidation forms acetyl-CoA which enters the Krebs cycle, but it is during oxidative phosphorylation that a significant amount of ATP is generated.