If you were hiring someone, what would be your key question you would ask them and why? What answers are you looking to hear? What are you hoping they don't say? This question is not job specific and not meant to be a brain teaser. It can be a situational question, behavioural question, psychological etc.
There are no real right or wrong answers to this question. It's a question to learn how the applicant thinks.
I find these questions very interesting in interviews and can qualify an applicant much better than learning what they studied for 10 years at Harvard.