Explanation
Correct answer: C.
Choice C is the best answer. The convention being tested is the use of punctuation within a sentence. This choice correctly uses a comma after “protest” to mark the boundary between the main clause (“For...protest”) and the supplementary appositive (“a decision...possessed”) that provides additional information about Anderson’s choice of narrator. Furthermore, the comma after “that” pairs with the comma after “own” to separate the supplementary adverbial phrase “by allowing...own” from the rest of the appositive. This adverbial phrase functions to explain how Anderson’s choice of a narrator amplified the agency and power the hospital workers possessed, 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 it results in a rhetorically unacceptable sentence fragment beginning with “a decision.”
Choice B
Choice B is incorrect because a semicolon can’t be used in this way to join the main clause (“For...protest”) and the supplementary appositive (“a decision...possessed”). A semicolon is conventionally used to join two main clauses, and “a decision...possessed” isn’t a main clause.
Choice D
Choice D is incorrect because it fails to mark the boundary between the main clause (“For...protest”) and the supplementary appositive (“a decision...possessed”) with appropriate punctuation.