Question

16 hair, nails, and the outer layer of the skin are made mostly of a tough waterproof protein called: a elastin b carotene c collagen d keratin

Explanation:

Keratin is a tough, fibrous protein that provides structure and protection. It is the main component of hair, nails, and the outer - layer of the skin, giving them their strength and waterproofing properties. Elastin is related to elasticity in tissues, carotene is a pigment, and collagen provides strength and structure in connective tissues but is not the main component of hair, nails, and outer - skin layer like keratin.

Answer:

D. keratin