Explanation
Correct answer: D.
Choice D 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 ("The poem…exclamation") and another sentence that begins with a supplementary element ("Translated…poem"). The supplementary element "translated as ‘hark!’ or ‘listen!’ in some versions" modifies the subject of the second sentence, "the word" (referring to hwæt).
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a comma splice after "exclamation." A comma can’t be used in this way to mark the boundary between sentences.
Choice B
Choice B is incorrect. Without a comma preceding it, the conjunction "and" can’t be used in this way to join sentences.
Choice C
Choice C is incorrect because it results in a comma splice after "versions." A comma can’t be used in this way to mark the boundary between sentences.