Okay, let's tackle this question. The user is asking about the most important vitamin that prevents autooxidation of polyunsaturated fatty acids (PUFAs) in cell membranes. The options aren't provided, but I need to figure out the correct answer based on my knowledge.

First, I remember that autooxidation of PUFAs is a process where free radicals cause oxidative damage, leading to lipid peroxidation. This can damage cell membranes and contribute to various diseases. Antioxidants play a key role in preventing this.

Vitamins that act as antioxidants include Vitamin C, Vitamin E, and Vitamin A. However, Vitamin E is specifically known for its role in protecting cell membranes. The reason is that Vitamin E (alpha-tocopherol) is a lipid-soluble antioxidant. It's located in the lipid bilayer of cell membranes where it can intercept free radicals before they attack PUFAs.

Vitamin C is water-soluble and works in the aqueous environment, but not directly in the membrane. Vitamin A is also fat-soluble but its primary role is in vision and immune function, not specifically protecting PUFAs. Vitamin K is involved in blood clotting and bone health, so probably not relevant here.

So the correct answer should be Vitamin E. The options might have been A. Vitamin A, B. Vitamin B, C. Vitamin C, D. Vitamin E. Then D would be correct. I need to structure the explanation accordingly, making sure to explain why Vitamin E is right and others are wrong. Also, mention the mechanism of action, like how it donates electrons to free radicals, preventing chain reactions.

**Core Concept**
The question tests knowledge of antioxidant vitamins that protect polyunsaturated fatty acids (PUFAs) in cell membranes from oxidative damage. Key to this is **Vitamin E (alpha-tocopherol)**, a lipid-soluble antioxidant that neutralizes free radicals in the membrane's lipid bilayer.

**Why the Correct Answer is Right**
Vitamin E prevents **lipid peroxidation** of PUFAs by acting as a **chain-breaking antioxidant**. It donates an electron to lipid radicals, stabilizing them and halting the oxidative chain reaction. This is critical in cell membranes rich in PUFAs, such as nerve and muscle cells. Unlike water-soluble antioxidants (e.g., Vitamin C), Vitamin E is uniquely positioned in the lipid environment to combat autooxidation.

**Why Each Wrong Option is Incorrect**
**Option A: Vitamin A** – While fat-soluble, its primary role is in vision and epithelial integrity, not membrane antioxidant defense.
**Option B: Vitamin C** – A water-soluble antioxidant; cannot access lipid membranes to prevent PUFA oxidation.
**Option C: Vitamin B complex** – B vitamins are coenzymes in metabolism, not antioxidants.

**Clinical Pearl / High-Yield Fact**
Remember **"E for membrane, C for aqueous"** – Vitamin E protects lipid membranes, while Vitamin C acts in extracellular fluids. Deficiency in Vitamin E causes **hemolytic anemia** due to RBC membrane damage from unchecked lipid peroxidation.

**Correct Answer: D. Vitamin E**

✓ Correct Answer: A. Tocopherol