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 (“Defying…fast”) and another sentence that begins with a supplementary phrase (“During…Olympics”).
Why the other choices are wrong
Choice A
Choice A is incorrect. When a dash is present in a sentence (“ran—fast”), it’s not conventional to use another dash (“fast—during”) to mark the boundary between sentences because it creates a potentially confusing sentence. In this context, a period, semicolon, or colon would be clear and more conventional.
Choice B
Choice B is incorrect because it results in a run-on sentence. The sentences (“Defying…fast”) and (“during…Olympics”) 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.