Question

in a relaxed skeletal muscle, actin is bound to
troponin only.
tropomyosin and troponin only.
tropomyosin, troponin, and myosin.
myosin only.

Explanation:

In a relaxed skeletal muscle, the active - sites on actin are covered by tropomyosin which is held in place by troponin. Myosin cannot bind to actin in this state. So actin is bound to tropomyosin and troponin only.

Answer:

tropomyosin and troponin only.