Which of the following vitamins is not classified as water-soluble?

Vitamin A is classified as a fat-soluble vitamin, which distinguishes it from the other options that are all water-soluble. Fat-soluble vitamins, such as A, D, E, and K, are stored in the body's fatty tissues and liver, and they require dietary fat for absorption. They are not excreted in urine like water-soluble vitamins, which means they can accumulate in the body and potentially lead to toxicity if consumed in excessive amounts.

In contrast, water-soluble vitamins, including Vitamin C and the B vitamins like B12 and folic acid, dissolve in water and are not stored in significant amounts within the body. They are readily excreted, which necessitates a regular intake through diet to maintain adequate levels. Understanding this difference is crucial for nutritional guidance and dietary planning.