Final answer:
The correct formula for the compound formed between magnesium and the phosphate ion is Mg3(PO4)2, because this combination balances the +2 charge of magnesium and the -3 charge of the phosphate ion.
Step-by-step explanation:
To determine the correct formula for the compound formed between magnesium and the phosphate ion, we must account for the charges on the magnesium ion (Mg2+) and the phosphate ion (PO43-). In order to make a neutral compound, the total positive charge must balance the total negative charge. The magnesium ion has a +2 charge, and the phosphate ion has a -3 charge. The least common multiple of 2 and 3 is 6. This means we need three magnesium ions to provide a +6 charge and two phosphate ions to provide a -6 charge. Therefore, the correct formula is Mg3(PO4)2, which means the answer is (c).