The passage is a set of facts so we would not usually diagram it. It tells us that outbreaks of mosquito born diseases typically happen after wet weather. One exception though is when mosquitos breed in wetland habitats, then outbreaks are worse after a period of drought. The question asks us to explain this exception.
Answer Choice C tells us that wetland habitats contain aquatic insects that feed on mosquito larvae, hence in a period of drought there are less aquatic insects that pray on mosquito larvae, and more mosquito larvae are able to survive and turn into adult disease carrying mosquitos. Since wetlands are a special case with lots of predators preying on mosquito larvae, it is an exception to the general rule.