The correct answer is option B, that is, the release of calcium ions makes actin and myosin to combine with each other.
At the time of the stimulation for a muscle contraction, calcium is discharged into the fiber. It combines with troponin and instigates it to move tropomyosin, exposing the binding site for the myosin cross-bridge. Thus, the face of the actin filament is now free to combine with myosin heads to initiate contraction of the muscle.