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.