Question

Sol. Key concept: Redox coenzymes differ from acyl-group carriers.

Vitamin B₂ (riboflavin) is the precursor of FMN and FAD. These flavin coenzymes accept and donate electrons or hydrogen in oxidation–reduction reactions.

Vitamin B₃ (niacin) gives NAD⁺ and NADP⁺, whose reduced forms NADH and NADPH participate in electron transfer. Thus the standard pair for redox coenzymes is B₂ and B₃.

Vitamin B₅ (pantothenic acid) is a component of coenzyme A. Its principal coenzyme role is transfer of acyl groups, rather than acting as the redox carrier described here.

Correction / clarification: The official key specifies (a), B₂ and B₅, but the scientifically appropriate pair B₂ and B₃ is absent. None of the printed options is fully correct as worded. Keep the official answer separate from the biochemical explanation. [R3]