## Core Concept
Glutathione is a tripeptide antioxidant produced in cells and plays a crucial role in maintaining cellular redox balance. It acts as an antioxidant by scavenging free radicals and reactive oxygen species (ROS). The antioxidant activity of glutathione depends on its reduction-oxidation (redox) state, which is maintained by the enzyme glutathione reductase and the availability of reducing equivalents.

## Why the Correct Answer is Right
The correct answer, **C. Vitamin C**, is related to the regeneration of reduced glutathione (GSH) from its oxidized form (GSSG). However, the direct cofactor for the enzyme glutathione reductase, which maintains glutathione in its reduced state, is NADPH. But among the vitamins, **Vitamin C (Ascorbic Acid)** helps in the regeneration of GSH by donating electrons to reduce oxidized glutathione (GSSG) back to GSH through the action of enzymes like dehydroascorbate reductase. This process ensures that glutathione can continue to act as an antioxidant.

## Why Each Wrong Option is Incorrect
- **Option A:** Vitamin A is crucial for vision, immune function, and skin health but is not directly involved in the antioxidant activity of glutathione.
- **Option B:** Vitamin E is a lipid-soluble antioxidant that protects cell membranes from oxidative damage but does not directly participate in the glutathione antioxidant system.
- **Option D:** Vitamin B12 plays a critical role in the synthesis of DNA, fatty acid, and amino acid metabolism but is not directly related to glutathione's antioxidant activity.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that **Vitamin C** not only helps regenerate glutathione but also acts as a water-soluble antioxidant in its own right, protecting cells from oxidative damage. Deficiencies in Vitamin C can impair the body's antioxidant defenses, making it a critical nutrient for overall health.

## Correct Answer Line
**Correct Answer: C. Vitamin C**