In muscle cells, myosin molecules continue moving along actin molecules as long as:

a. ADP is present and tropomyosin is released from intracellular stores.
b. ATP is present and troponin is not bound to Ca2+Ca2+.
c. ADP is present and the intracellular acetylcholine level is high.
d. ATP is present and the intracellular Ca2+Ca2+ concentration is high.

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.

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