Step 1: Fat-soluble vitamins.
Fat-soluble vitamins are vitamins that dissolve in fat and are stored in the liver and fat tissues. The fat-soluble vitamins are:
\[
\text{Vitamin A}, \text{Vitamin D}, \text{Vitamin E}, \text{Vitamin K}.
\]
Step 2: Vitamin C.
Vitamin C, also known as ascorbic acid, is a water-soluble vitamin, not a fat-soluble one. It dissolves in water and is not stored in the body.
Step 3: Conclusion.
Thus, the vitamin that is NOT fat-soluble is Vitamin C.
Final Answer: Vitamin C.