This is an easy question. You get asked this one all the time. From experience, I can tell you the best thing to do is say, wow, that's a great question. I'm not a hundred percent certain of the answer, so let's find out together. Or just simply give me a moment so that I don't misspeak. Clients love the humility when you can admit you don't know everything, but more importantly that you're willing to search for the correct answer instead of just blow smoke at them.