Final answer:
Retinoic acid is the active form of vitamin A and is used by the body without conversion. Beta-carotene is a precursor, not the active form, of vitamin A. Vitamin C is a water-soluble vitamin that is not stored in the body in significant amounts.
Step-by-step explanation:
Active Form of Vitamin A
The active form of vitamin A is Retinoic acid. This is because vitamin A functions in the human body in several forms, one of which is Retinoic acid (Vitamin A3), which is the acid form of Vitamin A and is biologically active. Beta-carotene is a precursor to vitamin A and undergoes conversion in the liver of humans through a dioxygenase reaction to form Retinal, also known as Vitamin A1. Retinal can then be further reduced to the alcohol form, Retinol, which is also active vitamin A. However, Retinoic acid is directly used in the body without the need for conversion. On the other hand, Calcitriol is an active form of vitamin D, and Tocopherol is another term for vitamin E.
Water-Soluble Vitamin
Regarding water-soluble vitamins, vitamin C is the correct answer. This vitamin, also called ascorbic acid, is not stored in the body in significant amounts and needs to be regularly consumed in the diet. Fat-soluble vitamins like vitamin A, vitamin E (Tocopherol), and vitamin K are stored in the body's tissues.