You can find a list of duties on a job description. This should help you to understand if they want to hire a secretary, on office manager, or something between these two. It should be easier for you to identify their goal and talk about right duties in your answer.

After all, your attitude matters the most. Show them that you are ready to work hard and do more than expected of you.

Sample answer:

"I believe that the main responsibility of an administrative assistant is to take care of all administrative and other assigned duties, so the managers can focus on the important tasks and carry out their job well. I believe that a good assistant should try to create a motivating and positive atmosphere in an office too."