Answer: The stage that produces most of the ATP during cellular respiration is the electron transport system (ETS).
Explanation: The inner mitochondrial membrane holds the ATPase enzyme and the complexes for ETS. In mitochondria, an electrochemical gradient is produced by the function of ETS, between the matrix and inner mitochondrial space(High at internal mitochondrial space and low at matrix). ETS gets electrons from the oxidation of NADH and FADH2. The ATPase enzyme situated in the inner mitochondrial membrane allows protons to flow down their electrochemical gradient. This proton motive force is utilized for the phosphorylation of ADP into ATP. Thus, the production of ATP through ETS is called oxidative phosphorylation as oxidation and phosphorylation occur.