The role of calcium in muscle contraction is
A. To break the cross bridges as a cofactor in the hydrolysis of ATP
B. To bind with troponin, changing its shape so that the actin filament is exposed
C. To transmit the action potential across the neuromuscular junction
D. To re-establish the polarization of the plasmas membrane following an action potential

Hint:Calcium ions play an important role in muscle contraction by building interactions between proteins, myosin and actin.

Complete answer:
The muscle is found in most animals as a soft tissue. Muscle cells produce actin and myosin protein filaments that move through each other and result experience a contraction that alters the cell's length and shape. Muscles work to build power and movement. Their primary role is to sustain and alter postures, locomotives, as well as internal organ movement, like heart contraction and the movement of food through peristalsis through the digestive system. There are three types of muscles. They are smooth muscle, skeletal, and cardiac.

In muscle contraction, calcium ions play an important role by building interactions between proteins, myosin and actin. The $Ca^{2+}$ ions bind to the C portion of the actin filament, exposing the binding region to which the myosin head is attached to stimulated muscle contraction. During muscle contraction, the displacement of troponin and the activity at which myosin binds to the power stroke involves high calcium concentrations. When the T-tubular membrane is excited, calcium is released from the sarcoplasmic reticulum through calcium ion channels. It binds with Troponin C and thus helps the myosin head to tighten onto the actin filament and to begin contraction of the muscles.
Thus, the correct answer is option B. i.e., To bind with troponin, changing its shape so that the actin filament is exposed.

Note:Calcium binds to the protein compound troponin and exposes active-binding sites to the actin activate the muscle contraction cycle. Then ATP binds to myosin, transfers myosin to its highly energy-intensive state and releases myosin head from the active actin site.