Here's why:

* Glycolysis: This initial step breaks down glucose into pyruvate, producing a small amount of ATP (2 molecules) and NADH.

* Krebs Cycle (Citric Acid Cycle): This cycle further breaks down pyruvate, generating some ATP (2 molecules), NADH, and FADH2.

* Oxidative Phosphorylation: This is the final stage where the electron carriers NADH and FADH2, generated in the previous steps, donate electrons to the electron transport chain. This process creates a proton gradient across the mitochondrial membrane, which is used by ATP synthase to generate a large amount of ATP (around 34 molecules).

Therefore, oxidative phosphorylation, with its large ATP yield, is the most energy-producing step of respiration.