What fibrous protein is hair chiefly composed of?

Hair is chiefly composed of keratin, which is a type of fibrous protein that plays a critical role in providing structure and strength to the hair shaft. Keratin is resilient and forms protective layers in hair, skin, and nails. This protein is also responsible for the durability and resistance of hair to various environmental factors, including heat, chemicals, and physical wear.

Collagen is primarily found in the skin, tendons, and ligaments, contributing to their strength and elasticity, but it does not significantly contribute to the structure of hair itself. Keratinin is not an accurate term recognized in hair biology; the proper term is keratin. Elastin is another fibrous protein that provides elasticity to tissues but is not relevant to the composition of hair. Consequently, keratin is the key fibrous protein that makes up the structure of hair, making it the correct answer.