Common sense, coherently and rationally. There's no "IRAC" alternative for a policy question, if that is what you're looking for. What the professor is looking for when asking a policy question is your ability to communicate and be persuasive for your point of view. Don't try to copy what you believe (or even know) that the professor thinks about the issue, it's just going to piss him off. He didn't spend a semester to make you a copycat.