Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is the use of verb forms within a sentence. The nonfinite present participle “prioritizing” is correctly used to form a supplementary element that modifies “their design,” describing what the headrests’ design prioritized.
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a comma splice. The finite past perfect verb “had prioritized” creates a second main clause in the sentence, and the comma after “headrests” can’t be used in this way to join two main clauses.
Choice B
Choice B is incorrect because it results in a comma splice. The finite past progressive verb “was prioritizing” creates a second main clause in the sentence, and the comma after “headrests” can’t be used in this way to join two main clauses.
Choice C
Choice C is incorrect because it results in a comma splice. The finite past tense verb “prioritized” creates a second main clause in the sentence, and the comma after “headrests” can’t be used in this way to join two main clauses.