Answer:
The correct answer would be the proton gradient.
Oxidation of NADH and FADH₂ release high energy electrons which are passed through the electron transport chain.
Some of the energy of these free electrons are used to form a proton gradient across the inner membrane of mitochondria in which the enzyme ATP synthase is placed.
The protons from the inter-membrane space pass down the concentration gradient through ATP synthase.
The movement of protons (charged particles) down the concentration gradient produces energy some of which is used by ATP synthase in order to phosphorylate ADP into ATP.
