In the respiratory electron transport chain cytochrome b is reduced by:

Explanation

The correct answer is coenzyme Q because it directly donates electrons to cytochrome b within Complex III of the respiratory electron transport chain. When coenzyme Q is reduced to ubiquinol, it carries electrons from Complex I and Complex II and transfers them to cytochrome b as part of the Q-cycle. This process is essential for maintaining the flow of electrons and generating the proton gradient that drives ATP synthesis. Hence, coenzyme Q plays the key role in reducing cytochrome b.NADH is incorrect because it transfers electrons to Complex I, not directly to cytochrome b.FADH₂ is incorrect because it donates electrons to Complex II, and from there the electrons move to coenzyme Q before reaching cytochrome b.Cytochrome c is incorrect because it is positioned later in the chain and receives electrons from Complex III after cytochrome b has already been reduced.