**Core Concept**
Vitamin E is a lipid-soluble antioxidant that plays a crucial role in protecting cells from oxidative damage caused by free radicals. It acts as a chain-breaking antioxidant, scavenging free radicals and preventing them from damaging cellular membranes.

**Why the Correct Answer is Right**
Vitamin E, specifically tocopherol, is the primary antioxidant responsible for protecting cell membranes from lipid peroxidation. It achieves this by donating a hydrogen atom to free radicals, thereby neutralizing them and preventing further chain reactions. Vitamin E is incorporated into cell membranes, where it can effectively scavenge free radicals and protect against oxidative stress.

**Why Each Wrong Option is Incorrect**
**Option A:** Niacin is a water-soluble vitamin that plays a crucial role in energy metabolism, but it is not primarily known for its antioxidant properties.
**Option B:** Vitamin A is a fat-soluble vitamin, but its primary role is in vision, immune function, and gene expression, rather than as an antioxidant.
**Option C:** Vitamin K is a fat-soluble vitamin essential for blood clotting, but it does not have significant antioxidant properties.

**Clinical Pearl / High-Yield Fact**
Vitamin E deficiency can lead to increased susceptibility to oxidative stress, which may contribute to various diseases, including atherosclerosis and neurodegenerative disorders.

**Correct Answer: C. Vitamin E.