**Core Concept**
Vitamins are organic compounds that are essential for various bodily functions, including energy production, immune function, and antioxidant defense. Antioxidants are molecules that neutralize free radicals, which are unstable molecules that can cause oxidative stress and cell damage.

**Why the Correct Answer is Right**
Among the vitamins, **Vitamin E** is a well-known antioxidant. It plays a crucial role in protecting cell membranes from oxidative damage by scavenging free radicals. Vitamin E's antioxidant properties are attributed to its ability to donate electrons to free radicals, thereby neutralizing them and preventing cellular damage. This is particularly important in maintaining skin health, preventing cancer, and reducing the risk of age-related diseases.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Vitamin A is primarily involved in vision, immune function, and skin health, but it is not an antioxidant.
**Option B:** This option is incorrect because Vitamin B complex includes several vitamins (B1, B2, B3, etc.) that are involved in energy production and nerve function, but none of them are antioxidants.
**Option C:** This option is incorrect because Vitamin K is essential for blood clotting and bone health, but it does not have antioxidant properties.

**Clinical Pearl / High-Yield Fact**
Vitamin E is an essential antioxidant that helps protect cells from oxidative damage, which can contribute to various diseases, including cancer, Alzheimer's disease, and atherosclerosis.

**Correct Answer:** D. Vitamin E.