Which vitamin is a fat-soluble vitamin?

Vitamin D is classified as a fat-soluble vitamin because it dissolves in fats and oils and is absorbed along with dietary fats in the intestine. Fat-soluble vitamins, which also include vitamins A, E, and K, are stored in the body's fatty tissues and liver, which allows the body to maintain reserves that can be utilized when dietary intake may be insufficient. This characteristic is essential for functions such as maintaining calcium levels in the blood, promoting bone health, and supporting immune function.

In contrast, the other choices represent water-soluble vitamins. Vitamin C is a water-soluble vitamin that plays a vital role in the synthesis of collagen and functions as an antioxidant. Thiamine (Vitamin B1) is also water-soluble, essential for energy metabolism and nervous system health. Folic acid, another water-soluble vitamin, is crucial for DNA synthesis and cell division, especially during periods of rapid growth. These vitamins do not get stored in large amounts in the body, and excess amounts are typically excreted in urine, highlighting the distinct roles that fat-soluble vitamins like Vitamin D play within human health.