In this question interviewer is checking your flexibility to work with business needs. One has to demonstrate willingness to work in a new area as well as desire to learn new things. You can support your answer by telling an example from the past experience.
Sample answer is: I am always seeking opportunities to learn new things. If it is a new area related to what I work on, then I will happily take this opportunity. Even in the past I have done projects on security domain. This helped me learn the nitty gritty of security aspects of an application.
If the new area were totally unrelated to my area of expertise, then I would need some time to learn the area. I will read about the area and gain more information on it. Also in this case, I will be open to management and set expectations in the beginning that I would need training on the new area to ramp up faster. This will help in delivering the work on time.