First, lets decide the charge of the ions, Calcium and Chloride. We must look at the periodic table to find both of them, and once we look at it, we can decide that Calcium has a 2+ charge and Chloride has a 1- charge. From here, we find out that for every Calcium that we have, we must have 2 Chloride's to cancel out the 2+ charge and therefore the answer is:
D)
