Explanation
Correct answer: C.
Choice C is the best answer. The phrase “making…at the time” provides additional information about Cahokia that’s not required for the sentence to make sense or function grammatically. As a nonessential supplement, this phrase should be separated from the rest of the sentence with a comma.
Why the other choices are wrong
Choice A
Choice A is incorrect. This choice results in a run-on sentence. The nonessential descriptive aside “making…at the time” needs to be separated from the rest of the sentence with a comma.
Choice B
Choice B is incorrect. This choice results in a sentence fragment. “Making…at the time” doesn’t have a subject and can’t stand on its own as a sentence. Thus, it can’t be separated from the rest of the sentence with a period.
Choice D
Choice D is incorrect. This choice results in a punctuation error. “Making…at the time” doesn’t have a subject and can’t stand on its own as an independent clause. Since a semicolon can only link two independent clauses, using one here creates an error.