I like what @lucillelucillelucille wrote. Also, Look at people in your life whom you admire and see how they deal with similar issues. Sometimes, learning from the social pros is a good way to go. :)
For a quick response, pretend you are thinking about it and give yourself a minute to compose yourself, and then either respond with “I don’t know, what do you think” or the Emily Post/Miss Manners all-time favorite “I don’t know…why do you ask?” Some people don’t like questions turned on them and may just drop the questioning. Because frankly, if someone is asking you a question which is personal and really none of their business, they know at some level what they are doing may be making you uncomfortable and you have a right to express that to them.