Final answer:
The correct formula for the ionic compound containing Al³⁺ and PO₄³⁺ is AlPO₄, which is aluminum phosphate, corresponding to option b.
Step-by-step explanation:
The correct formula for the ionic compound containing Al³⁺ and PO₄³⁺ is AlPO₄. When combining ions to form an ionic compound, the total positive charge must balance the total negative charge so that the compound is electrically neutral. Aluminum ion has a charge of +3 and phosphate ion has a charge of -3. Therefore, one aluminum ion can balance one phosphate ion, resulting in the formula AlPO₄, which is option b. This compound is known as aluminum phosphate.