Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is the use of punctuation within a sentence. This choice uses a semicolon in a conventional way to join the first main clause ("Gitlin’s…content") and the second main clause beginning with a supplementary phrase ("in…audiences"). Further, placing a comma after "era" separates the supplementary phrase "in that era" from the rest of the main clause that follows ("even…audiences").
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a comma splice. Without a conjunction following it, a comma can’t be used in this way to join two main clauses. Further, this choice fails to mark the boundary between the supplementary phrase "in that era" and the rest of the main clause that follows ("even…audiences").
Choice C
Choice C is incorrect because it results in a run-on sentence. The two main clauses ("Gitlin’s…content" and "in…audiences") are fused without punctuation and/or a conjunction.
Choice D
Choice D is incorrect because it results in a comma splice. Without a conjunction following it, a comma can’t be used in this way to join two main clauses.