Explanation
Correct answer: C.
Choice C is the best answer. The convention being tested is the use of punctuation and verb forms within a sentence. This choice leaves the verb "admired" in its nonfinite past participle form to function within a supplementary element ("much…followed"). Offset by commas after "works" and "followed," this supplementary element interrupts the main clause ("English poet and Shakespeare contemporary John Donne’s works…had…been essentially gathering dust…") with additional information about the works’ reception during Donne’s lifetime.
Why the other choices are wrong
Choice A
Choice A is incorrect because it fails to offset the supplementary element ("much…followed") with appropriate punctuation, and using the finite verb "were much admired" results in an ungrammatical sentence.
Choice B
Choice B is incorrect because using the finite verb "were much admired" results in an ungrammatical sentence.
Choice D
Choice D is incorrect because it fails to offset the supplementary element ("much…followed") with appropriate punctuation, and using the finite verb "had been much admired" results in an ungrammatical sentence.