The majority of NADH that delivers high-energy electrons to the electron transport chain comes from glycolysis and the citric acid cycle (Krebs cycle).

Here's a breakdown:

* Glycolysis: This process breaks down glucose into pyruvate, producing a net gain of 2 ATP molecules and 2 NADH molecules per glucose molecule.

* Citric Acid Cycle: This cycle further breaks down pyruvate, generating 3 NADH molecules per pyruvate molecule (6 NADH per glucose molecule).

While other metabolic pathways also contribute a small amount of NADH, the vast majority comes from these two central pathways of cellular respiration.