Which of the following vitamins is soluble in water?

Vitamins are classified by solubility into two groups: fat-soluble vitamins (A, D, E, K), which dissolve in lipids and are stored in the body's fatty tissues and liver; and water-soluble vitamins (the B-complex group and vitamin C), which dissolve in water and, with the exception of vitamin B12 (stored substantially in the liver), are not stored in significant amounts — any surplus is typically excreted in urine, so most must be replenished regularly through diet.

Applying this classification to the options: vitamin C (ascorbic acid) is a water-soluble vitamin, so it dissolves in water. It is the only water-soluble vitamin among the four options given.

• Vitamin D is fat-soluble; it is synthesized in the skin on sun exposure and stored in the liver and adipose tissue, not dissolved in water.

• Vitamin A is fat-soluble; it is stored in the liver and requires dietary fat for absorption in the intestine.

• Vitamin E is fat-soluble; it acts as a lipid-based antioxidant embedded in cell membranes and body fat stores.

Because excess vitamin C is excreted through urine rather than stored in fat tissue, its water solubility is confirmed, matching standard references on vitamin classification.