In the respiratory electron transport chain coenzyme - Q is reduced by:

Explanation

The correct answer is NADH. In the respiratory electron transport chain, coenzyme Q is reduced by electrons that originate from NADH through the action of Complex I, also known as NADH dehydrogenase. During this process, NADH donates a pair of high-energy electrons to Complex I, which are then transferred to coenzyme Q, reducing it to ubiquinol. This reaction is crucial for maintaining the continuous flow of electrons through the chain, helping establish the proton gradient that drives ATP synthesis. Therefore, NADH serves as the main source of reducing power for coenzyme Q in the electron transport system.FADH₂ is incorrect because it does not directly reduce coenzyme Q; instead, it transfers electrons to Complex II, which then passes them to coenzyme Q. In contrast, NADH donates electrons through Complex I, directly leading to the reduction of coenzyme Q.Cytochrome a is incorrect because it functions later in the chain, acting within Complex IV to transfer electrons to oxygen rather than reducing coenzyme Q.Cytochrome c is incorrect because it operates downstream of coenzyme Q, shuttling electrons between Complex III and Complex IV, and thus does not take part in the reduction of coenzyme Q.