OFFICIAL EXPLANATIONThe argument presented here is that homeowners endanger sparrows when they use termite repellent. This is because termites absorb the toxin in the repellent, becoming poisonous to any sparrow that may prey on them. It would be useful to know whether sparrows are actually eating these termites before we conclude that homeowners "put sparrows at risk" just by using termite repellent. Choice (A) may be useful to a homeowner trying to get rid of termites, but the performance of "other types of termite repellent" does not relate to the effects of this type of repellent on the sparrow population.
Choice (B) may be tempting, but even if termites lose their toxicity eventually, it doesn't change the fact that the sparrow may still be in danger if it eats the toxic termites now.
Choice (C) fits that prediction. If sparrows favor other sources of food, it's hard to claim that "the entire sparrow population" is in danger because a homeowner used this termite repellent. So, it would be useful to know whether these toxic termites are in fact their main source of food.
Choice (D) is irrelevant; we're interested in the sparrows' health here, not the termites'. Similarly, were not interested in the health of the "other insects" mentioned in (E).
The correct answer is Choice (C).