Name the ion responsible for unmasking of active sites for myosin for cross-bridge activity during muscle contraction. Sodium, Calcium, Magnesium, Potassium

Tropomyosin threads are present around an actin filament and troponin is there at intervals along the tropomyosin. The Troponin makes the Tropomyosin wrap around the actin filament in such a way so that it blocks the myosin binding with actin. But after the binding of calcium ions (coming from the sarcoplasmic reticulum), troponin changes its configuration. Because of this tropomyosin unblocks actin-myosin binding site on actin protein. Therefore, the correct answer is option C.