Cytochrome b is reduced by:

Explanation

In the electron transport chain, cytochrome b is reduced by receiving electrons from coenzyme Q (ubiquinone) at complex III. This reduction is crucial for the continued transfer of electrons through the chain. Cytochrome c, cytochrome a, and NADH play different roles in the electron transport chain. Cytochrome c receives electrons from cytochrome b but does not reduce it. Cytochrome a is part of complex IV and is involved later in the process. NADH donates electrons to complex I, not directly to cytochrome b.