This question is a great opportunity for you to show your potential for development and career progression. By proving that you understand the roles and responsibilities of your superiors, you are helping to create the impression that you have the potential to fit into that position at some point in the future.
To answer this question you could try to draw from your own experiences. If you have had some particularly strong supervisors it may be possible to identify which characteristics made them suitable for the position. Alternatively, if you have had poor experience of supervisors in the past, you could answer this question with the qualities you believe they were lacking.
If you can draw from your own experience your answer will sound more genuine and you will increase the chances of delivering a more fluid and confident response. However, if after thinking about this you are still drawing a blank, here are some examples to help you on your way:
I expect my supervisor to be at least as dedicated to successfully completing tasks as I am. A string work ethic from a supervisor always transmits through to the workers below him and it is always encouraging to come to work and see enthusiasm in your superiors. From experience I have always found a better working atmosphere when the supervisor has an open door policy. Being approachable helps with the subordinates feeling that any concerns will be heard and I usually find that morale will increase because of this.
I believe it is important for a supervisor to be open to thoughts and ideas from the workforce. It can be frustrating when a good idea is stifled because it did not come from the supervisor. When people believe that there ideas will be given consideration and are therefore important, I have found that this is likely to cause an increase in production and morale throughout the team.