Skin is the largest organ of the body and it uses its exposure to sunlight for a very special purpose.
Sebum (A) is the waxy/oily secretion of sebaceous glands of the skin, which lubricates hair and skin, and makes the skin “waterproof”.
Keratin (B) is the most abundant fibrous structural protein found in nails or hair. It protects epithelial cells from stress and damage.
Vitamin A (C) is derived from carotenoids consumed in diet. It is considered a very important antioxidant and plays critical role in vision.
Therefore, options A, B and C are incorrect.
Vitamin D (D) is a fat-soluble vitamin, which is present in very small amounts in consumed foods. It can be produced within the skin by exposure to sunlight. Vitamin D helps in calcium and phosphate metabolism.
Therefore, option (D) is correct.