The best response to this question will be 'yes' given the nature of the career and the industry. That said, if you have barriers to relocation, then consider what those barriers are and whether or not they might ever have the flexibility to change. The interviewers are looking for candidates that will be ready to step in and fill where the needs exist, so be sure to emphasize your desire to be open and flexible regardless of your answer. The majority of candidates will say 'yes,' so be sure to really sell your answer with either a story or personal motivation for your willingness to relocate. Note that if you are not willing to relocate at any point for this position, then this might not be the career for you.