Hi emilyjmyer!
Happy to address why answer choice (C) is incorrect.
To begin, this question asks about the effects of a pond drying up in June, which should bring to mind material from the passage: "But small ponds occasionally dry up during the summer, forcing the water bugs to search for new habitats, an eventuality that macropterous individuals are well adapted to meet" (lines 39-42). We also know from the same paragraph that the "summer (early) generation ... is usually dimorphic" (lines 32-33). We're therefore told that the "dimorphism of the summer generation would enable some individuals to survive," as stated in answer choice (D). Namely, this dimorphism means that there are some macropterous individuals, and these can survive a pond drying up. Or represented in conditional reasoning, we are given:
dimorphism 
macropterous
And answer choice (D) adds:
macropterous 
survive pond drying up
We can put these together as:
dimorphism macropterous survive pond drying up
Or:
dimorphism survive pond drying up
To address answer choice (C), you mention,
I feel like the question stem makes it seem like it is an unusually warm June.
I can understand why you might be inclined to think this from the question stem. After all, if the water bed is dried up, would this not suggest it was especially hot? Despite the temptation, it's important not to put more into the questions than is actually there. For example, the pond might have dried up because a city started directing the pond's water elsewhere. Or perhaps it was an especially dry summer with no rain, yet wasn't necessarily hot. In short, we don't know from the wording of the question that it was especially hot in June.
Finally, even if it were hot in June, that wouldn't make (C) the correct answer. Answer choice (C) states, "The generation of water bugs to be hatched during the subsequent spring would contain an unusually large number of macropterous individuals." We're only told about the conditions that produce macropterous individuals in the final sentence of the passage: "Adult water bugs of the overwintering generation, brought into the laboratory during the cold months and kept warm, produce only macropterous offspring" (lines 63-66). To be sure, this connects heat and macropterous individuals, but with reference to water bugs that are adults in June (if heat dried up the pond, some adults could survive because of their macropterous wings). The role of heat in the passage, however, is more with respect to egg development. We'd want to know about the heat conditions of the
eggs mentioned in answer choice (C) to know if they'd produce macropterous individuals.
