Explanation
Correct answer: C.
Choice C is the best answer. “By contrast” logically signals that the information about bathymetric maps in this sentence—that they show landforms’ elevation below the sea—presents a direct contrast to the previous information about topographic maps showing landforms’ elevation above sea level.
Why the other choices are wrong
Choice A
Choice A is incorrect because “in conclusion” illogically signals that the information about bathymetric maps in this sentence concludes or summarizes the previous information about topographic maps. Instead, the sentence provides contrasting information about bathymetric maps.
Choice B
Choice B is incorrect because “for example” illogically signals that the information about bathymetric maps in this sentence exemplifies the previous information about topographic maps. Instead, the sentence provides contrasting information about bathymetric maps.
Choice D
Choice D is incorrect because “afterward” illogically signals that this sentence describes an event that chronologically follows the previous information about topographic maps. Instead, the sentence provides contrasting information about bathymetric maps.