(B) Al₂O₃
Al₂O₃ is an amphoteric oxide — it reacts with HCl to form AlCl₃ + H₂O, and with KOH to form KAlO₂ + H₂O. CuO, Na₂O, and K₂O are basic oxides; they react with acids only.
The key concept is amphoteric oxides — oxides that react with both acids and bases. Al₂O₃ is the classic example at the Class 10 level. CuO, Na₂O, and K₂O are purely basic oxides, so they react with HCl but not with KOH. Examiners expect you to recall that amphoteric oxides show dual behaviour.