Explanation
Correct answer: C.
Choice C is the best answer. The convention being tested is the use of verb forms in a sentence. The nonfinite past participle phrase "highly prized" is correctly used to form a supplementary element that modifies the main clause "this…artistry," describing memorable features of Brass Era automotive design.
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a comma splice. Using the finite present tense verb phrase "are highly prized" creates a second main clause in the sentence, and two main clauses can’t be joined in this way with only a comma after "artistry."
Choice B
Choice B is incorrect because it results in a comma splice. Using the finite past perfect tense verb phrase "had been highly prized" creates a second main clause in the sentence, and two main clauses can’t be joined in this way with only a comma after "artistry."
Choice D
Choice D is incorrect because it results in a comma splice. Using the finite past tense verb phrase "were highly prized" creates a second main clause in the sentence, and two main clauses can’t be joined in this way with only a comma after "artistry."