How can I move my answer up from an A to an A*?

If you are working at an A standard you have demonstrated you have the knowledge; the main difference between an A and an A* lies in how you present that information.

Make sure you offer an answer to the question, clearly stating what you think and convincingly backing up your answer rather than just listing what other people have argued.

Give a sense of evaluation; rather than listing answers or facts indicate which you think is the most important reason and why.