Explain the central difference between main details and supporting details of a piece of writing.

Main detail is the main point of your paragraph/essay. you want to be able to start and finish with that so it will remain within the reader thought for longer.
(We Brits have a saying "if a piece of literacy is stuck in your head, the writer has accomplished his or her goal" )
the supporting details are smaller points that correlate and support the main point. note that you should never start with a supporting point, nor should you ever end with one, it is simply there to extend and highlight the main detail. starting or ending with supporting details will ensure that the reader will almost instantaneously forget your essay and they may even find it boring because it doesn't show consistency.  

Your answer should be “b”