Final answer:
The electron transport chain produces the most ATP through oxidative phosphorylation, especially when compared to glycolysis and fermentation, which yield much less ATP. The correct option is a & c.
Step-by-step explanation:
The process that produces the most ATP is the electron transport chain (ETC). While glycolysis and fermentation are processes that yield energy, they produce a minimal amount of ATP. Glycolysis converts glucose to pyruvate, producing a small amount of ATP. If oxygen is available, pyruvate enters the mitochondria, where it is further broken down by the Krebs cycle to produce NADH and FADH2. These electron carriers then provide electrons to the ETC, which creates a proton gradient used to produce a significant amount of ATP through oxidative phosphorylation. Oxygen is required for the ETC to function, thus in the presence of oxygen, the combination of the Krebs cycle and ETC produces much more ATP than glycolysis or fermentation alone.