Explanation
Correct answer: C.
Choice C is the best answer. The convention being tested is the use of punctuation within a sentence. The dash after “vibrato” pairs with the dash after “abilities” to separate the supplementary element (“acquiring...vibrato”) from the rest of the sentence. This supplementary element functions to describe the vocal abilities Vaughan developed, and the pair of dashes indicates that this element could be removed without affecting the grammatical coherence of the sentence.
Why the other choices are wrong
Choice A
Choice A is incorrect because a colon can’t be paired with a dash in this way to separate the supplementary element from the rest of the sentence.
Choice B
Choice B is incorrect because a comma can’t be paired with a dash in this way to separate the supplementary element from the rest of the sentence.
Choice D
Choice D is incorrect because it fails to use appropriate punctuation to separate the supplementary element from the rest of the sentence.