The Correct Answer or the Most Helpful One?

If you can't answer both ways, I think the "correct" answer is the right way to go. That way you ensure you answer their question, even if it's not in the most helpful way possible. When you offer alternatives you're not necessarily giving them what they need, and it's your fault not theirs. If the question is so badly worded that they get a useless answer which happens to be correct, that's their fault.