I'm assuming you're applying for the full three year nursing course, in which case you could say that you see yourself installed on a ward or in the community or whatever.
You could also say what aspect of nursing particularly interests you at the moment, and that you see yourself working in that area, but that you hope your training will help you determine whether or not it's the right area for you. A friend, for instance, had a desire to specialise in cardiac nursing, but has ended up in respiratory work after a particularly successful placement there.
Another thing to think about is that in 5 years' time you may well be looking towards a more advanced qualification in whatever will be your specialism, or possibly find yourself moving towards a teaching or training role.
As buildersmate says, the question is primarily designed to test how much you have really thought about what you're doing, and to make sure this application isn't just something that seemed like a good idea at the time. At the end of the day they want committed people.