How to answer this common interview question?

I would say something along the lines of I look forward to continuing me education and possibly pursuing a masters degree but am looking for a unit that I can call home for the next few years until that time. Managers typically don't expect a new grad will stay on the same unit for 30 years until they retire. They know realistically you won't be there forever but you want to show them that you aren't just going to jump ship in 6 months either.

You could reframe the question and change the area of focus to the "area of nursing practice" you most enjoy. For instance, it may be patient/family education or assessment/diagnosis, etc. That would apply to all sorts of areas rather than locking you down to only one. For me - the most satisfying part of my clinical practice has always been encountering new things - researching the topic/diagnosis/treatment, "putting all the pieces together" to figure out what is (likely to) happen".