As a tutor, you will predominantly be helping GCSE and A-Level students with their exams. This means you will often be helping students prepare for exam-style questions, and covering the course content to go alongside this.
Your sample Q&A should therefore be an example of a common question asked by GCSE or A-level students. As well as being asked to run through this with your interviewer to get a sense of your tutoring style, your subject Q&A will appear on your profile for parents and students to see. You are welcome to upload more than one example.
Feel free to have a look at what other tutors have written in the past.