Explanation
Correct answer: D.
Choice D is the best answer. "Specifically" logically signals that the information in this sentence—that the researchers examined the overhead costs and attendance at events of 22,328 nonprofits—provides specific, precise details elaborating on how the researchers investigated the relationship between nonprofits’ spending and performance.
Why the other choices are wrong
Choice A
Choice A is incorrect. "Thus" illogically signals that the information in this sentence is a result or consequence of the researchers’ investigation of nonprofits’ spending and performance. Instead, it specifies how they examined that correlation.
Choice B
Choice B is incorrect. "In addition" illogically signals that the information in this sentence is merely an additional fact about the researchers’ investigation of nonprofits’ spending and performance. Instead, it specifies how they examined that correlation.
Choice C
Choice C is incorrect. "By comparison" illogically signals that the information in this sentence is being compared to the researchers’ investigation of nonprofits’ spending and performance. Instead, it specifies how they examined that correlation.