Correct Answer: B
Explanation:
The charge of an oxide anion is -2 and the charge of an aluminum cation is +3. The smallest number that is evenly divisible by both 2 and 3 is 6. Therefore you will need two aluminum cations for a total of +6 and three oxide anions for a total of -6 in order for the charges to balance out.
A clever way to arrive at the correct formula is to use the charge of each element as the subscript of the other element...note how that works for this example!