Humans were long thought to have begun occupying the Peruvian settlement of Machu Picchu between 1440

Choice A 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 one sentence (“However...earlier”) and another (“In...area”). The supplementary phrase “in 1420 CE” modifies “humans,” the subject of the third 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 C is incorrect because it results in a comma splice. A comma can’t be used in this way to mark the boundary between sentences. Moreover, the subordinating conjunction “which” creates a confusing and illogical sentence that suggests that the supplementary phrase beginning with “in” modifies the previous information (“However...earlier”) rather than the information that follows. Choice D is incorrect because it results in a run-on sentence. The sentences (“However...earlier” and “in...area”) are fused without punctuation and/or a conjunction.