Where can keratin be found in humans?

Keratin is a type of fibrous protein that plays a crucial role in providing structure and protection to various tissues in the body. It is most notably found in hair, nails, and the outer layer of skin. This protein is responsible for the strength and resilience of these structures, making option B the correct choice.

Keratin helps form the protective barrier that prevents damage from environmental factors, such as physical abrasion and pathogens. In hair, keratin provides the necessary strength and flexibility, while in fingernails, it contributes to their hardness and durability.

The other options do not appropriately represent locations where keratin is found. Blood plasma contains a variety of proteins, but keratin is not one of them. Bones and muscles consist primarily of collagen and other proteins, rather than keratin. Lastly, while the respiratory system has various proteins and tissues, it does not typically involve keratin in its structures. This makes the identification of hair and fingernails as the primary sites of keratin accumulation accurate and supported by its biological function.