What is needed for the myosin head to attach to the binding site on the actin molecule?
a. atp
b. sarcomere
c. calcium
d. sodium

The correct answer will be option-C.

Explanation:

The muscle contraction occurs as a result of the sliding of the myofilaments made up of thin actin filaments and thick myosin filaments. The actin contains a binding site for the myosin which is covered with the tropomyosin-troponin complex proteins.  

The troponin contains a binding site for the "calcium" which is released from the sarcoplasmic reticulum in response to the nerve signals. The calcium binds the troponin and causes conformation changes in tropomyosin which exposes the myosin-binding sites to bind myosin. The myosin binds and forms cross-bridge and cause muscle contraction.

Thus, Option-C is the correct answer.

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