Assure the interviewer that if they granted you the opportunity, you'd plan to stay long term with Marsh McLennan. How you answer this question could inform them of the level of your ambition and where you aim to take your career.
Every employer aspires to invest their resources in those interested in mutual growth towards like-minded opportunities. Interviewing, hiring, and onboarding is an expensive and time-consuming process striving towards that aim. While they pass on those whom they feel are on the hunt to land a job anywhere, with no genuine interest in mutual growth, a hiring manager has a vested interest to ensure you're aspiring to grow your career with them. One way of assessing your interest is by determining the measure of your career goals and how they align with what they have to offer.
A career minded individual who takes charge of their own development typically has a five-year career plan, and has written out their core values and their mission statement. This practice makes it easier for a professional to identify the right match, understand their motivators, and the type of work environments that enhance their productivity and drive their level of success. This will help them in their job search to find like-minded opportunities that provide a symbiotic match for an employer/employee relationship.