Be honest, but try to show evidence for careful consideration and logic in your choice. The interviewer may follow up this question by asking if, with hindsight, you felt you made the right choice, or how you feel the course you have followed could be improved. Constructive and thoughtful criticism is appreciated, but try to balance any criticisms with positive points too.
You could include in your answer any of the following:
* Advice & guidance from teaching/careers staff at school or college * Geographical preferences * Availability of a particular course * The research and/or teaching quality of institutions * Cost factors * The reputation of the institution, or individual members of staff
