Explanation
Correct answer: A.
Choice A is the best answer. "By contrast" logically signals that the information in this sentence—that isolating molybdenum was a relatively long process—contrasts with the previous information that isolating selenium was a short process.
Why the other choices are wrong
Choice B
Choice B is incorrect because "thus" illogically signals that the information in this sentence is a result of the previous information about selenium’s short isolation process. Instead, the sentence presents contrasting information about molybdenum’s longer isolation process.
Choice C
Choice C is incorrect because "similarly" illogically signals that the information in this sentence is similar to the previous information about selenium’s short isolation process. Instead, the sentence presents contrasting information about molybdenum’s longer isolation process.
Choice D
Choice D is incorrect because "for instance" illogically signals that the information in this sentence exemplifies the previous information about selenium’s short isolation process. Instead, the sentence presents contrasting information about molybdenum’s longer isolation process.