Explanation
Correct answer: D.
Choice D is the best answer. "Specifically" logically signals that the information in this sentence—Latimer making the light bulb more durable—provides a specific detail elaborating on the more general claim in the previous sentence that Latimer improved the light bulb.
Why the other choices are wrong
Choice A
Choice A is incorrect because "soon" illogically signals that the information in this sentence occurred shortly after Latimer improved the light bulb. Instead, Latimer making the bulb more durable was the specific improvement.
Choice B
Choice B is incorrect because "regardless" illogically signals that the information in this sentence is true despite the previous claim about Latimer. Instead, the information about Latimer making the bulb more durable provides a specific detail elaborating on that claim.
Choice C
Choice C is incorrect because "however" illogically signals that the information in this sentence contrasts with the previous claim about Latimer. Instead, the information about Latimer making the bulb more durable provides a specific detail elaborating on that claim.