Explanation
Correct answer: C.
Choice C is the best answer. "On the other hand" logically signals that the information in the sentence—that the snowpack at higher elevations in the Chuska Mountains was deepest in mid-March—contrasts with the previous information about the snowpack at lower elevations being deepest in early March.
Why the other choices are wrong
Choice A
Choice A is incorrect because "in other words" illogically signals that information in the sentence is merely a paraphrase or restatement of the previous information about the snowpack at lower elevations. Instead, the information about the snowpack at higher elevations contrasts with that information.
Choice B
Choice B is incorrect because "for instance" illogically signals that the information in the sentence exemplifies the previous information about the snowpack at lower elevations. Instead, the information about the snowpack at higher elevations contrasts with that information.
Choice D
Choice D is incorrect because "in summary" illogically signals that the information in the sentence summarizes the previous information about the snowpack at lower elevations. Instead, the information about the snowpack at higher elevations contrasts with that information.