Explanation
Correct answer: A.
Choice A is the best answer. “By contrast” logically signals that the information in this sentence—that paper scraps left over from printing a magazine are pre-consumer waste—contrasts with the previous information about magazines being post-consumer waste.
Why the other choices are wrong
Choice B
Choice B is incorrect because “for example” illogically signals that the information in this sentence exemplifies the previous information about post-consumer waste. Instead, the paper scraps being pre-consumer waste contrasts with the previous information.
Choice C
Choice C is incorrect because “as a result” illogically signals that the information in this sentence is a result or consequence of the previous information about post-consumer waste. Instead, the paper scraps being pre-consumer waste contrasts with the previous information.
Choice D
Choice D is incorrect because “specifically” illogically signals that the information in this sentence provides specific, precise details elaborating on the previous information about post-consumer waste. Instead, the paper scraps being pre-consumer waste contrasts with the previous information.