Explanation
Correct answer: C.
Choice C is the best answer. “However” logically signals that the theory discussed in this sentence—that humans may have arrived in the Americas over 30,000 years ago—contrasts with the previously discussed theory that humans arrived around 13,000 years ago.
Why the other choices are wrong
Choice A
Choice A is incorrect because “as a result” illogically signals that the theory in this sentence is the result of the theory discussed in the previous sentence. Instead, this theory contrasts with the previous one.
Choice B
Choice B is incorrect because “similarly” illogically signals that the theory in this sentence is similar to the theory discussed in the previous sentence. Instead, this theory contrasts with the previous one.
Choice D
Choice D is incorrect because “in conclusion” illogically signals that the theory in this sentence concludes or summarizes the discussion of the previous theory. Instead, this theory contrasts with the previous one.