Within a prestigious field like investment banking, you should have strong personal motivation to succeed in your work, and your interviewer is asking this question to see what makes you tick in your career. In your answer, you might include an anecdote that explains why you got into IB in the first place. Perhaps a previous manager has even commented on your motivation, which you should certainly include in your answer.

While money, top-notch benefits, and the reputation that come with a career in investment banking may be all motivators, don't focus on these aspects in your answer. Rather, focus your answer on intrinsic motivation mostly removed from external factors like money, status, or external validation.