The interviewers are looking for your experience level working with certain clients. This question likely indicates that the open position will need to work with a specific client type. When preparing a response to this question, outline each of the client types you've worked with and rank them by frequency. Review the posted job description for clues as to what client type this position will work with and compare that to your list. If you have little experience or haven't worked directly with that client type, then research transferrable correlations between your 'frequent client list' with the target client type of the position and include the correlation in your response.