I may be wrong, but I think it's Antibodies.
Answer:
Calcium.
Explanation:
Muscle cells are the important for the body as they are involved in the bodily movement. Three different type of muscle are smooth muscle, cardiac muscle and skeletal muscle.
The sarcoplasmic reticulum contains the stored calcium in the muscle cell. The calcium is important ion that plays an important role in the muscle contraction. The actin and myosin protein functioning also depends on the the calcium ion.
Thus, the correct answer is option (B).
