The answer is C. It states that Mark is the only one who can run effortlessly. Choice A is wrong because it has no punctuation before "Mark". Choice B is wrong because in order to use a semicolon effectively, the joining statement must be able to stand on it's own and "Mark" cannot be it's own sentence. Choice D is wrong because it sounds like someone is speaking to Mark about how only one of the runners seems to run effortlessly. The comma completely changes the sentence.
