Which of the following proteins primarily contributes to the contraction of muscle fibers?

Myosin is the protein that plays a central role in the contraction of muscle fibers. It is a motor protein that interacts with actin, another essential protein in muscle contractions. Myosin has a unique structure that enables it to convert chemical energy stored in ATP into mechanical energy, generating movement. During muscle contraction, the myosin heads bind to actin filaments, pulling them closer together, which shortens the muscle fiber.

Actin, while also critical to muscle contraction, primarily serves as the framework or track for myosin to act upon. It works in tandem with myosin, but it is myosin that provides the energy and force that drives the contraction process. Keratin, on the other hand, is a structural protein found in hair, nails, and the outer layer of skin, and it does not play a role in muscle fiber contraction. Tubulin is the protein that makes up microtubules, which are involved in cell structure, transport, and division but are not directly involved in muscle contraction. Thus, myosin's specific role as a motor protein directly responsible for muscle contraction makes it the correct answer to the question.