I’m a paragraph of no less than 125 words, explain the single most important factor involved in advancing american classical music in the twentieth century.

This sounds like a personal reflection on something you did. it is asking the hardest part, or a part you may have struggled with on an informal interview you must have done. it is also asking advice you'd give someone else for the process of completing an informal interview, so you should write something that would . for example, maybe the first thing they should do, something they should include.