I was curious about this too. What about the context tells me this is "you" and not "they"? I mean I would assume that THEY would not eat the other apple; apple being singular, but it's not nonsensical like some of the other questions that have been provided here.
Given that there is no real context provided I have noticed that the site will allow either translation, very nice considering it can be either one. Also note that if Sie is capitalized in the middle of a sentence it is automatically a formal you rather than they.
Du isst den anderen Apfel is correct, but Ihr esst den anderen Apfel is also correct, so if you only chose the first one, it is marked as a wrong answer.