Final answer:
When NAD gains two electrons and a hydrogen ion, it is reduced, transforming from NAD+ to NADH. correct option is a.
Step-by-step explanation:
When NAD reacts with hydrogen and gains two electrons, it is reduced. This is because reduction in chemistry refers to the gain of electrons by a molecule. Specifically, in this process, Nicotinamide adenine dinucleotide (NAD+) gains two electrons and a hydrogen ion (proton), transforming into its reduced form, NADH.
The oxidized form of this electron carrier is NAD+, which does not contain the extra electrons and proton. However, upon reduction, the form changes to NADH, wherein the nitrogenous base in NADH has one more hydrogen ion and two more electrons than in NAD+.
Therefore, the correct answer to the question 'When NAD reacts with hydrogen and gains two electrons, is it reduced, oxidized, or hydrolyzed?' is (a) Reduced.