My GC says to always answer questions like that, unless you have an ethical problem with it. So, just go with your gut and don't worry about how it will affect your chances. Saying that you're white will certainly not help increase your chances, but then again, neither will leaving the question blank. They're not going to assume that you're a URM if you leave the question blank--if anything, they'll assume that you're white or Asian and don't want to tell them, or simply ignore that part of your app. In short: probably answering the question is best.