Which vitamin is a precursor to NAD+?

Niacin, also known as vitamin B3, is the direct precursor to NAD+ (Nicotinamide adenine dinucleotide). In metabolic processes, niacin can be converted into NAD+ through a series of biochemical reactions. NAD+ is crucial for various biological functions, including redox reactions and energy metabolism, making niacin an essential nutrient for maintaining cellular health. Riboflavin plays a role in the production of other coenzymes but is not a direct precursor to NAD+. Folate is important for DNA synthesis and repair, while pyridoxine functions primarily in amino acid metabolism. Thus, niacin's unique role as a precursor makes it the correct answer.