The interviewer is checking to see that it’s not because you are unemployable. The best way to answer this question is to be truthful. Whether it’s a career break to raise children or looking after a sick relative, travelling the world or you returned to university, answer with honesty. Practice a short-concise response without over indulging personal issues which could derail the interview.
A good answer could be:
I returned to university to complete my Master’s degree in Project Management and I passed with a distinction. It’s something I been wanted to accomplish for a while. Now that the course is complete I am focused on my career; refreshed and eager to put my knowledge into practice.