The hover hint says "qual" can mean "which," but "Which is the excuse" is not an acceptable answer.
On hover, mine says "what" as well as "which". In this context, "what" is the correct meaning. As a sentence "Which is the excuse?" would not make sense in most situations.
Sure, within context, usually "qual" in "Qual é a desculpa?" would mean "what," but image a child telling his mummy about something wrong he did and he came up with a different excuse every time she asks why he did it. Thus, "Which is the excuse?," na? Point being, I wish there were more context, but that's not really the strong point of duolingo's format. But don't get me wrong, I love duolingo--I haven't been able to learn this much Portuguese despite wanting to for months.