Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is punctuation use between sentences. In this choice, the period after “configurations” is used correctly to mark the boundary between one sentence (“The intense…configurations”) and another (“TMAO…fish”). The supplementary phrase (“ensuring…configurations”) modifies the main clause of the first sentence (“The chemical…effect”), and “TMAO” is the subject of the second sentence.
Why the other choices are wrong
Choice B
Choice B is incorrect because it results in a run-on sentence. The sentences (“The intense…configurations” and “TMAO…fish”) are fused without punctuation and/or a conjunction.
Choice C
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.
Choice D
Choice D is incorrect. Without a comma preceding it, the conjunction “and” can’t be used in this way to join sentences.