Explanation
Correct answer: C.
Choice C is the best answer. "However" logically signals that the claim about reindeer in this sentence contrasts with the information about most deer species in the previous sentence.
Why the other choices are wrong
Choice A
Choice A is incorrect because "similarly" illogically signals that the claim in this sentence is similar to the previous information about most deer species. Instead, it contrasts with that information.
Choice B
Choice B is incorrect because "next" illogically signals that the claim about reindeer in this sentence is the next step in a process. Instead, it contrasts with the previous information about most deer species.
Choice D
Choice D is incorrect because "thus" illogically signals that the claim in this sentence results from the previous information about most deer species. Instead, it contrasts with that information.