Final answer:
The two components that let muscles contract are actin and myosin filaments. Together with regulatory proteins tropomyosin and troponin, these filaments interact as part of the sliding filament model of muscle contraction.
Step-by-step explanation:
The two components of muscle that allow it to contract are actin and myosin filaments, which correspond to choice a) Actin; myosin. The thick filaments in muscle fibers are composed of myosin, which is primarily responsible for the force exerted during muscle contraction. These thick filaments interdigitate with thin filaments, made mostly of actin, which contain binding sites for myosin attachment. The interaction between these filaments is fundamental to the mechanism of muscle contraction.
In addition to actin and myosin, thin filaments also include the regulatory proteins tropomyosin and troponin. Tropomyosin strands block myosin binding sites on actin when the muscle is at rest, and troponin, which binds calcium ions, helps to initiate the contraction process by moving the tropomyosin strands away from the binding sites. The sliding filament model of muscle contraction involves actin filaments being pulled by myosin filaments upon a signal from the nervous system, leading to muscle shortening and thus contraction.