In the respiratory electron transport chain cytochrome a is reduced by the oxidation of:

Explanation

The correct answer is cytochrome c because it directly transfers electrons to cytochrome a in Complex IV of the respiratory electron transport chain. Cytochrome c acts as a small, mobile electron carrier located on the outer surface of the inner mitochondrial membrane. After receiving electrons from Complex III, it donates them to cytochrome a and a₃, which together form cytochrome c oxidase. This electron transfer is essential for the final reduction of oxygen to water, making cytochrome c the immediate electron donor responsible for reducing cytochrome a.NADH is incorrect because it donates electrons at the very beginning of the chain to Complex I, not directly to cytochrome a.FADH₂ is incorrect because it transfers electrons through Complex II and coenzyme Q, far earlier in the pathway than cytochrome a.Coenzyme Q is incorrect because it passes electrons to Complex III, not to Complex IV where cytochrome a operates.