In the context of fibrous and globular proteins, what is the primary function of keratin in vertebrates?

• A. Transports oxygen in the blood via a heme-containing globular protein
• B. Catalyzes peptide bond hydrolysis as a digestive protease in the stomach
• C. Functions as an antibody that specifically binds antigens in the immune response
• D. Provides structural strength and protective support in tissues such as hair, nails, and the outer layer of skin

Answer: (D)