In some cascades, calcium acts via a calcium-binding protein

called (A) calcium-binding protein R7. (B) calmodulin. (C)

collagen. (D) cyclic AMP. (E) calcium-binding protein S4.

(B) calmodulin

Explanation:

Calcium in cells acts as second messenger molecule which regulates the very important physiological and cellular process of cell-like neurotransmission, cell migration, secretion, fertilization and many others.

Calcium acts as a messenger by binding to a calcium-binding protein called calmodulin. Calmodulin forms complex with 4 calcium ions through the side amino acids present in the structure of calmodulin.

Thus, calmodulin is the correct answer.