Explanation
Correct answer: B.
Choice B is the best answer. "Conversely" logically signals that the information in this sentence—that countries with proportional-representation electoral systems tend toward multi-partyism—contrasts with the previous information about countries with single-ballot majoritarian elections, which tend to have two-party systems.
Why the other choices are wrong
Choice A
Choice A is incorrect because "subsequently" illogically signals that the information in this sentence about countries with proportional-representation electoral systems occurs later in a chronological sequence of events than the information in the previous sentence. Instead, it contrasts with the previous information.
Choice C
Choice C is incorrect because "for instance" illogically signals that the information in this sentence about countries with proportional-representation electoral systems is an example supporting the previous statement about countries with single-ballot majoritarian elections. Instead, it contrasts with the previous statement.
Choice D
Choice D is incorrect because "in other words" illogically signals that the information in this sentence about countries with proportional-representation electoral systems is a paraphrase or restatement of the previous information about countries with single-ballot majoritarian elections. Instead, it contrasts with the previous information.