**Core Concept**
Glutathione is a tripeptide antioxidant that plays a crucial role in protecting cells from oxidative damage. It is composed of glutamine, cysteine, and glycine residues, and its antioxidant properties are mediated by the sulfhydryl group of cysteine.

**Why the Correct Answer is Right**
Glutathione requires vitamin B3 (Niacin) to act as an antioxidant. Niacin is essential for the synthesis of NADPH, which is required for the reduction of glutathione disulfide (GSSG) to its reduced form (GSH). This reduction process is crucial for maintaining the antioxidant capacity of glutathione. In the absence of NADPH, glutathione becomes oxidized, leading to a decrease in its antioxidant activity. **Vitamin C** can also reduce GSSG to GSH, but Niacin is the primary cofactor required for this process.

**Why Each Wrong Option is Incorrect**
**Option A:** Vitamin E is a lipid-soluble antioxidant that protects cell membranes from oxidative damage, but it is not directly involved in the reduction of glutathione disulfide.
**Option C:** Vitamin C can reduce GSSG to GSH, but it is not the primary cofactor required for this process. Vitamin C is also not essential for glutathione's antioxidant activity.
**Option D:** Vitamin A is a fat-soluble vitamin that plays a crucial role in vision, immune function, and reproduction, but it is not directly involved in glutathione's antioxidant activity.

**Clinical Pearl / High-Yield Fact**
Remember that Niacin is essential for the synthesis of NADPH, which is required for the reduction of glutathione disulfide. This is a critical process for maintaining glutathione's antioxidant capacity and protecting cells from oxidative damage.

**Correct Answer:**
✓ Correct Answer: B. Niacin