Explanation
Correct answer: C.
Choice C is the best answer. The convention being tested is the punctuation of a supplementary element within a sentence. The comma after “that” pairs with the comma after “roles” to separate the supplementary element “among their many roles” from the rest of the sentence. This supplementary element functions to clarify that polyphenols have many roles, and the pair of commas 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 dash can’t be paired with a comma to separate the supplementary element from the rest of the sentence.
Choice B
Choice B is incorrect because a semicolon can’t be paired with a comma to separate the supplementary element from the rest of the sentence.
Choice D
Choice D is incorrect because a colon can’t be paired with a comma to separate the supplementary element from the rest of the sentence.