Explanation
Correct answer: D.
Choice D is the best answer. "Instead" logically signals that the idea in this sentence—that researchers should use the word "lyfe"—is an alternative to the idea mentioned in the previous sentence (scientists’ use of the word "life").
Why the other choices are wrong
Choice A
Choice A is incorrect because "previously" illogically signals that the idea in this sentence occurs before the action in the first sentence. Instead, the use of "lyfe" is an alternative to the previously mentioned use of "life."
Choice B
Choice B is incorrect because "regardless" illogically signals that the idea in this sentence is true despite the information in the first sentence. Instead, the use of "lyfe" is an alternative to the previously mentioned use of "life."
Choice C
Choice C is incorrect because "there" illogically signals that the idea in this sentence occurs in a place mentioned in the previous sentence. Instead, the use of "lyfe" is an alternative to the previously mentioned use of "life."