Final answer:
Option A). A myosin head does not attach to actin at rest due to the absence of calcium ions. Cross-bridge formation occurs only when calcium ions bind to troponin, exposing the myosin binding sites on actin during muscle contraction. ATP attachment to myosin is necessary for any detachment that may occur.
Step-by-step explanation:
A myosin head is not joined to actin when it is at rest. Rather, it's in a ready state where a fresh ATP molecule binds to the myosin head, causing any possible cross-bridge to separate. Tropomyosin, to which the troponin complex is attached, covers the myosin binding sites on actin. This complex reacts with calcium. Only when the muscle is signaled to contract can cross-bridge formation occur because of the conformational change that calcium ions cause when they bind to troponin. This movement of tropomyosin exposes the myosin binding sites on actin. Actin active sites are shielded and no cross-bridge forms in the absence of calcium ions and a nerve impulse, which stops myosin from adhering to actin.