My choice is the first that came to mind, as explained in my previous posting. This context – or my first thought about this context – refers to the question of which scenario actually caused the Maya Kingdom to collapse.
I do think that b) would be a possibility in an appropriate context. The context would be a general reference to collapsed societies like the Maya Kingdom; in that case, one could say "could collapse" – it would be hard to imagine how any society like this could collapse.
As well, "collapsed" could be used, although it is not one of the choices. If "how such a society collapsed" is used, then the reference is to the fact of the collapse, not how the collapse came about, which is the case with "could collapse" and "could have collapsed."