Which of the following vitamins is usually classified as fat-soluble?

Vitamin A is classified as a fat-soluble vitamin because it dissolves in fats and oils and is stored in the body's fatty tissues and liver. Unlike water-soluble vitamins, which are excreted in urine and need to be consumed regularly, fat-soluble vitamins are absorbed more effectively in the presence of dietary fat. They play crucial roles in various bodily functions, including vision, immune function, and skin health.

In contrast, Vitamin C, folic acid, and niacin are all water-soluble vitamins. Water-soluble vitamins dissolve in water and are not stored in the body to a significant extent, requiring regular replenishment through the diet. Water-soluble vitamins generally are expelled through urine, making it important to consume them consistently in a balanced diet. This fundamental difference in solubility and storage highlights why Vitamin A is the correct answer in this context.