Explanation
Correct answer: C.
Choice C is the best answer. The convention being tested is punctuation use between a noun and a prepositional phrase. No punctuation is needed between the noun "pieces" and the prepositional phrase "of pottery." The prepositional phrase provides essential information about what kind of pieces were found, so it shouldn’t be separated from the rest of the noun phrase ("some of the oldest pieces") with punctuation.
Why the other choices are wrong
Choice A
Choice A is incorrect because no punctuation is needed between the noun phrase "some of the oldest pieces" and the prepositional phrase "of pottery."
Choice B
Choice B is incorrect because no punctuation is needed between the noun phrase "some of the oldest pieces" and the prepositional phrase "of pottery."
Choice D
Choice D is incorrect because no punctuation is needed between the noun phrase "some of the oldest pieces" and the prepositional phrase "of pottery."