The answer is A) thick filaments.
Myosin is a type of protein that plays a crucial role in muscle contraction and is a primary component of thick filaments in muscle fibers. Thick filaments are primarily made up of myosin molecules that aggregate to form the structural unit necessary for muscle contraction. In contrast, thin filaments are primarily composed of actin, along with other proteins like troponin and tropomyosin. Myofilaments refer to the collective term for both thick and thin filaments within the muscle, while Z discs are structures that anchor the thin filaments and define the boundaries of a sarcomere, but they do not contain myosin. Thus, myosin is solely associated with thick filaments.