Explanation
Correct answer: D.
Choice D is the best answer. “On the other hand” logically signals that the information in this sentence—that dust can lower sea surface temperatures, weakening the storm—presents an opposing effect to the one described in the previous sentence, in which dust enhances ice cloud formation and increases precipitation.
Why the other choices are wrong
Choice A
Choice A is incorrect because “previously” illogically signals that the information in this sentence is an event that occurred before the dust-enhanced ice cloud formation mentioned in the previous sentence. Instead, the dust lowering surface temperatures—and weakening the storm—is an opposing effect.
Choice B
Choice B is incorrect because “in other words” illogically signals that the information in this sentence is a paraphrase or restatement of the previous information about dust enhancing ice cloud formation. Instead, the dust lowering surface temperatures—and weakening the storm—is an opposing effect.
Choice C
Choice C is incorrect because “for example” illogically signals that the information in this sentence is an example of the previous information about dust enhancing ice cloud formation. Instead, the dust lowering surface temperatures—and weakening the storm—is an opposing effect.