Great question!

The key is the wording "under acidic conditions" which implies that you will use H+ to balance out hydrogens and H2O to balance out oxygens.

This is covered in ChemOChem Lesson 6: Solutions and Electrochemistry (Balancing Redox Reactions) on page 398 (based on my page numbers).

The solution suggests solving this question by elimination, which is a great technique in this case! Since alumina (Al2O3) has oxygen atoms, then we KNOW that we are going to need H2O to balance out the oxygen atoms. Aaaand the addition of H2O molecules will necessitate the addition of H+ ions. 😀 So, D is the only possible answer.