I think that this is a question of adaptability, the answer to which heavily relies on your own ability to retain multiple ideas involving more than one topic/theme depending on the type of piece that you will be writing.
If you feel that you would be more comfortable and confident with using one main topic/theme and expressing it through each type of writing, then you should probably adopt this approach, as you would gain a good ability to easily switch between types of writing if required and effectively incorporate your topic/theme into whichever type of piece you end up writing accordingly. This is especially the case if the Module C question ends up being fairly general.
On the other hand, if you feel that you would have no issues with preparing multiple topics and adapting those to each type of writing (for example, if you believe that dealing with more ideas would not be problematic for you) then you should probably adopt this approach. Given the unpredictable nature of Module C questions (with reference to whether a specific type of writing and/or a reflection is required), preparing multiple topics/themes and being able to manipulate those according to a particular type of writing may constitute more effective preparation and could also enable you to come up with ideas on the spot more easily if needed.
I hope this helps!
