This is a good question for a lot of prospective employers to ask you at the start of the interview, as it allows them to weigh up your motivations. Obviously, this question requires you to understand the job advert/spec inside and out, so have a good think about why you wanted to apply. There are lots of different reasons you could use; it could be that it was the company in particular or a specific aspect of the job description that you were interested in. That said; try not to use the salary as a reason, it just looks like you're only in it for the money, which is never a good sign for a prospective employer.