Welcome to the group @Lucky13!
Do you mind if I ask the context where you were asked?
My response depends a lot on the situation.
There can be a lot of reasons they might ask (some of them my fault because my concealment failed in some way). If its in a posted no gun location (signs have no force of law in MO, but they can ask you to leave, and that’s legal) I am probably going to answer, leave my shopping cart or whatever, ask to speak with the manager, and then explain why I won’t be doing business with them ever again (politely, and making clear that since they are not providing for my security, and they don’t want me to, I will take my money to someone who understands that the right to self protection is an American Constitutional Thing.)
Sometimes its a friend or acquaintance that has genuine questions, fears, concerns… that’s a teachable moment and I’ll have that conversation every day all day.
Sometimes it’s pure curiosity, and then I have to evaluate if this is a discussion I want to have with that person. I teach firearms and CC so I’m open to conversation with random strangers… they may become students, and occasionally I do have that conversation, along with giving them my card. This may happen at places like the gas pump (truck has a couple stickers that might be understood) or restaurants or shops.
If its someone who’s nervous about it in a non-posted location (say, grocery store) and I’ve been “spotted” I will say something like “Yes ma’am, I do carry, for your protection as well as my own.”
If I’m not in for the conversation, I may say “I think its best not to discuss personal subjects like firearms and politics with people I don’t know.” and pointedly change the subject “How 'bout them Cardinals?” or “Odd weather we’re having isn’t it?” If they are rude and aggressive, and I’m cornered for some reason and can’t just exit with a “have a nice day”, my very last resort is to ask “is there some reason you want to discuss personal topics that are not your business?” and then we can talk about why they want to be in my business, but I won’t be discussing firearms. I have once or twice had that rude confrontation turn into a teachable moment, but it takes time and they’re not usually interested in actual discussion, so its pretty darn rare.
Hope that helps!