Explanation
Correct answer: D.
Choice D is the best answer. "That is" logically signals that the information that follows in the sentence clarifies a term used in the previous discussion of the researchers’ findings: cognate replacement.
Why the other choices are wrong
Choice A
Choice A is incorrect because "for example" illogically signals that the information that follows in the sentence provides a specific example of the research findings described earlier. Instead, it clarifies a term introduced earlier in the sentence.
Choice B
Choice B is incorrect because "likewise" illogically signals that the information that follows in the sentence is similar to the previous information about the researchers’ findings. Instead, it clarifies a term introduced earlier in the sentence.
Choice C
Choice C is incorrect because "in addition" illogically signals that the information that follows in the sentence merely adds to the previous information about the researchers’ findings. Instead, it clarifies a term introduced earlier in the sentence.