Final answer:
NAD+ is a coenzyme derived from vitamin B3 (niacin) and serves as an electron acceptor in redox reactions, where it is reduced to NADH. It is not derived from vitamin E, does not release adenosine to become NADH, nor is it the reduced form of niacin.
Step-by-step explanation:
Among the options provided for NAD+, the one that is true is that it is a coenzyme. Specifically, NAD+ (nicotinamide adenine dinucleotide) is an oxidized coenzyme involved in redox reactions, where it serves as an electron carrier. It is derived from vitamin B3 (niacin) and plays a crucial role in metabolic processes. When NAD+ accepts two electrons and a proton, it gets reduced to NADH, which is the active form that can then donate electrons in subsequent biochemical reactions. Therefore, NADH represents the reduced form of the coenzyme. NAD+ does not come from vitamin E, release an adenosine to become NADH, or represent the reduced form of niacin. It does not serve as an electron source, but rather as an electron acceptor during oxidation reactions in the body.