The balanced reaction is:
4 Al + 3 O2 --> 2 Al2O3
Each Aluminum atom wants to lose 3 electrons to become isoelectronic with Oxygen. Each Oxygen on the other hand wants to gain 2 electrons. Since the reaction shows 6 Oxygen atoms in the product and each want to gain 2 electrons, therefore a total of 12 electrons are transferred. Since the problem ask for the electrons transferred per formula unit, hence:
6 electrons (ANSWER)