Explanation
Correct answer: C.
Choice C is the best answer. “For this reason” logically signals that the behavior described in this sentence is a consequence of the information about seals in the previous sentence. That is, a seal moves its flippers as little as possible during a deep dive because it needs to keep its heart rate low enough that it does not run out of oxygen.
Why the other choices are wrong
Choice A
Choice A is incorrect because “in the first place” illogically signals that this sentence is the first point in a discussion. Instead, the sentence describes a behavior that is a consequence of the previous information about seals.
Choice B
Choice B is incorrect because “on the other hand” illogically signals that the behavior described in this sentence contrasts with the previous information about seals. Instead, it is a consequence of that information.
Choice D
Choice D is incorrect because “in comparison” illogically signals that the behavior described in this sentence is being compared to the previous information about seals. Instead, it is a consequence of that information.