Which of the following acid is also a vitamin?

Vitamins are organic compounds required in small (trace) amounts for normal metabolism; several vitamins are, by chemical structure, carboxylic acids — Vitamin C's official chemical name is L-ascorbic acid.

Among the acids listed, only Ascorbic acid is officially classified and required in the diet as a vitamin (Vitamin C) — it is essential for collagen synthesis, wound healing, and antioxidant defence, and its absence causes the deficiency disease scurvy.

• Malic acid: a dicarboxylic acid naturally present in apples and used as a food acidulant and citric-acid-cycle intermediate; it is not classified as a vitamin.

• Palmitic acid: a saturated (C16) fatty acid common in animal fats and palm oil; it serves as an energy-yielding nutrient, not a vitamin.

• Formic acid: the simplest carboxylic acid, present in ant stings (its name derives from Latin "formica", ant) and used industrially; it has no recognized vitamin function in the human body.

Hence, Ascorbic acid is the acid that is also a vitamin.