Is your experience mainly with B2C or B2B Clients?
The interviewer is asking this question because they might work with either more B2B or B2C clients. They want to know what clients you have the most experience with and are the most comfortable working with.
I would focus on the clients that you have the most experience with no matter which one it is. If you would prefer more experience in one over the other then make sure to share that as well.
I would avoid saying anything negative about past clients. You may have had a bad experience but sharing that information won't be something that is going to help you move forward to the next stage of the interview process.
"I have worked with more B2C than B2B clients in my past roles. I enjoy working with B2C clients because I am a huge consumer myself so it's easier for me to relate. I also like seeing how my strategy impacts so many people around me and can see the direct impact almost immediately."