d. ATP is present and the intracellular Ca2+Ca2+ concentration is high.
Explanation:
Myosins are motor proteins that plays a major role in muscle contraction. They depend on ATP and responsible for motility in actin-based substances. Skeletal muscle is composed of myosin and actin fibers. Each myosin thick filament is surrounded by actin thin filaments, and each thin filament is surrounded by thick filaments. There exists a strong attraction between the myosin head and the myosin-binding site of actin which binds them spontaneously. This bond is aided by the presence of ATP and the high concentration of intracellular Ca2+. The myosin protein undergoes a change in its shape.
