Explanation
Correct answer: D.
Choice D is the best answer. "Though" logically signals that the claim in the sentence—that Morton’s improvisational skills helped shape jazz as a genre during its early years ("No one can deny" it)—is true despite the previous information about Morton’s exaggerated claim to have invented jazz.
Why the other choices are wrong
Choice A
Choice A is incorrect because "therefore" illogically signals that the claim in the sentence is a result of the previous information about Morton’s claim to have invented jazz. Instead, the sentence states that Morton helped to shape jazz—even if his claim was an exaggeration.
Choice B
Choice B is incorrect because "in the second place" illogically signals that the claim in the sentence is a second, separate point in addition to Morton’s claim to have invented jazz. Instead, the sentence states that Morton helped to shape jazz—even if his claim was an exaggeration.
Choice C
Choice C is incorrect because "in other words" illogically signals that the claim in the sentence is merely a paraphrase or restatement of the previous information about Morton’s claim to have invented jazz. Instead, the sentence states that Morton helped to shape jazz—even if his claim was an exaggeration.