"Besides apples, which fruit do you eat?"
Translation:Tranne le mele, quale frutta mangi?
Actually, I think the system would be right in this case: Using "tu" would be fine indeed, but it should have been: "quale frutta tu mangi"; not "quale frutta mangi tu". I think the syntax in Italian does not reverse when posing questions, in contrast to English for example. That's why your answer got marked as wrong.