How Does NAD⁺ Serve as a Catalyst in Oxidation Reactions

How does NAD⁺ serve as a catalyst in oxidation reactions?

A) It accepts a hydride ion at the 4-position of its pyridinium ring.
B) It accepts a hydride ion at the amide carbonyl group at the 3-position.
C) It accepts a proton at the 4-position of its pyridinium ring.
D) It accepts a proton at the amide carbonyl group at the 3-position.
E) none of the above