Explanation
Correct answer: A.
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 ("In…plant") and another ("By…weeds"). The prepositional phrase beginning with "by" modifies the subject of the next sentence, "she," which refers to Macías-Rubalcava.
Why the other choices are wrong
Choice B
Choice B is incorrect because it results in a comma splice. A comma can’t be used in this way to join two sentences.
Choice C
Choice C is incorrect. Without a comma preceding it, the conjunction "and" can’t be used in this way to join two sentences.
Choice D
Choice D is incorrect because it results in a run-on sentence. The two sentences are fused without punctuation and/or a conjunction.