Vitamin D is unlike any other vitamin because it actually functions as a hormone in the body. Once it`s produced in the skin in response to sunlight, it`s sent to the kidneys to be activated, and then functions in managing calcium in the blood, bones and gut and helping cells all over the body to communicate properly.

Option 1 (A): Vitamin A is important in boosting immunity and vision but it doesn`t act as a hormone.

Option 2 (B): Vitamin B complex includes many different vitamins that are not hormones but are vital for various biochemical processes, such as metabolism and nervous system function.

Option 3 (C): Vitamin C is necessary for the formation of collagen and the absorption of iron, but it doesn`t function as a hormone.

Hence the correct answer is Vitamin D (Option 4) which functions as a hormone in the body.