I do a lot of public speaking, presenting and demoing to audiences of various sizes, and am considered (by at least some people) quite good at this stuff.
My advice would be just stop and think when you need to. It's completely fine and you will convey the impression that you are a serious and thoughtful individual, which is good. You can show that you were listening and are about to respond entirely through your body language.
If you feel uncomfortable with that, there are a bunch of simple tricks you can use here and saying "umm..." is not a good one. One easy thing is to always ensure you have a glass/bottle of water. If someone asks a tough question and you need time to think, wait until they have finished their question, then take a drink of water. No-one thinks this is even slightly strange and it gives you time to think and make a considered answer.
My advice would be just stop and think when you need to. It's completely fine and you will convey the impression that you are a serious and thoughtful individual, which is good. You can show that you were listening and are about to respond entirely through your body language.
If you feel uncomfortable with that, there are a bunch of simple tricks you can use here and saying "umm..." is not a good one. One easy thing is to always ensure you have a glass/bottle of water. If someone asks a tough question and you need time to think, wait until they have finished their question, then take a drink of water. No-one thinks this is even slightly strange and it gives you time to think and make a considered answer.