Explanation
Correct answer: D.
Choice D is the best answer. “In addition” logically signals that the information in this sentence—that the Twentieth Amendment requires newly elected US senators and representatives to be sworn in on January 3—is separate from and additional to the amendment’s mandate concerning presidential inaugurations.
Why the other choices are wrong
Choice A
Choice A is incorrect because “instead” illogically signals that the information in the sentence presents an alternative to or substitute for the Twentieth Amendment’s mandate concerning presidential inaugurations. Rather, the sentence presents a separate requirement in addition to that one.
Choice B
Choice B is incorrect because “for instance” illogically signals that the information in the sentence exemplifies the Twentieth Amendment’s mandate concerning presidential inaugurations. Instead, the sentence presents a separate requirement in addition to that one.
Choice C
Choice C is incorrect because “specifically” illogically signals that the sentence provides specific, precise details elaborating on the Twentieth Amendment’s mandate concerning presidential inaugurations. Instead, the sentence presents a separate requirement in addition to that one.