Explanation
Correct answer: C.
Choice C is the best answer. “By contrast” logically signals that the information in this sentence—that dogs can see, hear, and smell by the end of two weeks—contrasts with the preceding information (that wolves can smell but not see or hear at the same age).
Why the other choices are wrong
Choice A
Choice A is incorrect because “in other words” illogically signals that the information about domesticated dogs in this sentence paraphrases the information about wolves in the previous sentence. Instead, the information about dogs contrasts with what came before.
Choice B
Choice B is incorrect because “for instance” illogically signals that the information about domesticated dogs in this sentence exemplifies the information about wolves in the previous sentence. Instead, the information about dogs contrasts with what came before.
Choice D
Choice D is incorrect because “accordingly” illogically signals that the information about domesticated dogs in this sentence is in accordance with, or results from, the information about wolves in the previous sentence. Instead, the information about dogs contrasts with what came before.