Which of the following acids is a vitamin?
A) Aspartic acid
B) Ascorbic acid
C) Adipic acid
D) All of these

Hint: Vitamins are organic compounds of diverse structure which are not used for energy or tissue framework, but are required in minute amounts for the normal functioning of living organisms. Dr. Casimir Funks (1911), for the first time found that the food, apart from ordinary nutrients, contains substances that are essential for normal functioning of the animal body called vitamins.

Complete answer:
Vitamins are ‘a group of organic substances which function catalytically in cellular metabolism’ or ‘accessory food factors’. Fresh fruits and vegetables are a rich source of vitamins. Vitamins are of two types – fat soluble (vitamin E, K, A and D) and water soluble (vitamin B1, B2, B3, B12, H, C, etc.).

Any hydrogen-containing substances which are capable of donating a hydrogen ion to another substance and can remain energetic are called acids. The litmus paper turns from blue to red and indicates that the substance is acidic in nature.

Now let us analyze the given options:
A) Aspartic acid- It is a non-essential amino acid that has a negative charge. They play an important role in the synthesis of the other amino acids, citric cycles and urea cycles. Also used in the biosynthesis of proteins.
B) Ascorbic acid - The generic name of this acid is (Vitamin C) and used to treat the low levels of vitamin C in those who do not get enough of the vitamin from their diet. Deficiency of vitamin C leads to scurvy. It is an important vitamin that is essential for the repairing of tissues.
C) Adipic acid – It is an organic compound and a most important dicarboxylic acid which is used as a precursor for nylon production.

So, the correct answer is option (B) Ascorbic acid.

Note: Plants can make vitamins from simple substances, but animals are unable to synthesize them. Animals must obtain vitamins from their food. Vitamins act as the constituent of coenzymes in the cells. Lack or shortage of any vitamin leads to malfunctioning of the body called deficiency disease.