What type of protein fiber is known for its strength and is commonly found in bones and tendons?

The protein fiber that is recognized for its exceptional strength and is a primary component found in bones and tendons is collagen. This structural protein provides support and structure to various tissues in the body, including skin, cartilage, and ligaments, in addition to bones and tendons. Collagen fibers are incredibly strong and resilient, allowing them to withstand significant tensile forces, which is critical for the mechanical properties required in these tissues.

Elastin, on the other hand, is primarily responsible for elasticity in tissues, allowing them to return to their original shape after stretching. While important, it does not provide the same level of strength as collagen.

Keratins are a group of protective proteins found in structures like hair, nails, and the outer layer of skin. Their role is more about protection and forming barriers rather than providing the tensile strength associated with bones and tendons.

Actin is a type of protein that plays a crucial role in muscle contraction and cell movement. It is not primarily involved in providing structural strength in the same manner as collagen does in connective tissues like bones and tendons.

Thus, collagen is the correct choice for its distinctive strength and structural role in various tissues throughout the body.