On sunny days, dark rooftops absorb solar energy and convert it to unwanted heat, raising the

Choice C is the best answer. The convention being tested is punctuation use between sentences. In this choice, the period is used correctly to mark the boundary between the first sentence (“On…temperature”) and the second sentence (“Adding…effect”). The gerund phrase beginning with “adding” is the subject of the second sentence, and the verb phrase “helps combat this effect” describes what adding a light-colored covering can do. 

Choice A is incorrect because a semicolon can’t be used in this way to join the sentence “On...temperature” and the supplementary phrases that follow. Doing so leaves the verb phrase “helps combat” without a subject and thus results in a grammatically unconventional sentence. Choice B is incorrect because it results in a comma splice. A comma can’t be used in this way to mark the boundary between sentences. Choice D is incorrect. This choice results in a confusing and illogical sentence that suggests that adding a light-colored covering to an existing dark roof raises the temperature of the surrounding air. Furthermore, it creates ambiguity by leaving the verb phrase “helps combat” without a subject (so it isn’t clear what helps combat the effect).