There is no right answer to this question, only wrong ones. You don’t need to make book out of this answer, just something short and positive is best. After all, it really does not matter to the interviewer, as long as you don’t say something foolish.

The point here is to convey to the interviewer that you are not leaving because you are mad, tired, bored, overworked, underpaid, or job hopping, just that you are leaving your job on because.