A. Vitamin - A vitamin is an organic molecule (or related set of molecules) that is an essential micro-nutrient that an organism needs in small quantities for the proper functioning of its metabolism. Ascorbic acid is a vitamin found in various foods such as spinach, cabbage, tomatoes, potatoes etc and sold as a dietary supplement. It is also called vitamin C. It is used to prevent and treat scurvy.
It is an essential nutrient involved in the repair of tissue and the enzymatic production of certain neurotransmitters. Hence, the ascorbic acid is a vitamin. B. Hormone - special chemical messengers in the body that are created in the endocrine glands. These messengers control most major bodily functions.
C. Enzymes - proteins concerned in the catalyzation of biochemical reactions. D. Amino acid - bio macromolecules contains an amino group, R group, CooH group and H.
So, the correct option is 'vitamin'.