Explanation
Correct answer: D.
Choice D is the best answer. “As a result” logically signals that the information in this sentence—that the widened grooves in balsa wood can accommodate electrical conductors—is a direct consequence of the researchers’ method described in the previous sentence (using heat and chemicals to widen the grooves).
Why the other choices are wrong
Choice A
Choice A is incorrect because “for example” illogically signals that this sentence provides an example of the researchers’ method described in the previous sentence. Instead, the sentence indicates a consequence of that method.
Choice B
Choice B is incorrect because “previously” illogically signals that this sentence describes an event that occurred before the researchers’ development of the method described in the previous sentence. Instead, the sentence indicates a consequence of that method.
Choice C
Choice C is incorrect because “in contrast” illogically signals that the information in this sentence contrasts with the previous sentence’s description of the researchers’ method. Instead, the sentence indicates a consequence of that method.