While giving the interview, one should keep in mind that the interviewer tries to ask questions that might have long term as well as short term repercussions.
If you fit well in the company, and if the company has a global presence, it might even be rudimentary for you to travel a lot or to relocate for a couple of years.
Of course, not everyone is open to relocation or travel, and therefore depend on the job requirements the company tries to find out if you are willing to relocate or whether you are willing to travel and how often.
This is a pretty simple question, and it entirely depends on your personal decisions (and the salary package).
However, if you are not comfortable answering this question, it is more than okay to inform the interviewer that you would like to take some time to answer the question or would be able to answer the question on a later date.
Actually speaking, the question whether you would be relocating or not is quite a rhetoric question, except if there is some special case.
If a person is married with kids, and the kids are of school age, the company itself will understand that the person would not prefer to relocate, as it would be quite a hassle for his family.
You should also understand the difference between traveling and relocation:
Traveling is basically going to a particular place and then returning within a day, week or even a month.
However, relocation simply means that you have to pack your bags and live wherever the company places you, with you being responsible for your family.
Therefore, it is always suggested that you do not answer the question before considering your family status and other financial issues.
