Explanation
Correct answer: B.
Choice B is the best answer. "As a result" logically signals that the information in this sentence—that many people in Switzerland gained a new opportunity to participate in their country’s political process—is a result or consequence of the previous information about the Swiss government lowering the voting age.
Why the other choices are wrong
Choice A
Choice A is incorrect because "nevertheless" illogically signals that the information that follows is despite the information in the previous sentence about the Swiss government lowering the voting age. Instead, people gained a new opportunity to participate in Switzerland’s political process as a result or consequence of that change.
Choice C
Choice C is incorrect because "by contrast" illogically signals that the information in this sentence contrasts with the information in the previous sentence about the Swiss government lowering the voting age. Instead, people gained a new opportunity to participate in Switzerland’s political process as a result or consequence of that change.
Choice D
Choice D is incorrect because "similarly" illogically signals that the information in this sentence is similar to the information about the Swiss government lowering the voting age in the previous sentence. Instead, people gained a new opportunity to participate in Switzerland’s political process as a result or consequence of that change.