When troponin binds calcium, a. it changes conformation, twisting tropomyosin and exposing actin-myosin binding sites. b. ion channels open and sodium rushes into the muscle cell. c. it allows tropomyosin to bind actin. d. it changes conformation, exposing the atp binding site and allowing the actin-myosin bond to break. e. atp binds to the myosin head, allowing myosin to attach to actin.

Option (A).

Explanation:

The muscle contains the different type of proteins that helps in the contraction of muscles and the movements of different body parts. The proteins are actin, myosin, nebulin and tropomyosin.

The binding of the troponin with the calcium result in the confrontational change. This twists the another protein tropomyosin. This helps in the exposure of the actin-myosin binding site. Later, the ATP acts as the main factor for the muscle contraction.

Thus, the correct answer is option (A).