Hi, I took AQA A-level geography and yes you will choose one topic for each component. It might be different for your exam board but for me I had to study water & carbon cycles for section A. Then for sections B it was a choice between coasts, deserts and glaciers. My school chose coasts and then for section C it was hazards. Your school will choose the optional topics to study. For the sections where there are optional topics, you only do the the one which you've been taught and ignore the others.