Final answer:
A two-party system is one where only two parties realistically have a chance at winning elections. The United States is an example of a two-party system due to its winner-take-all elections and historical political developments. This system often excludes third parties from gaining significant power.
Step-by-step explanation:
The two-party system is best defined as b) A system in which only two parties have a genuine chance at winning an election. In a two-party system, multiple political parties may exist, but only two major political parties dominate the political landscape and are realistically able to gain control of government offices. The United States is a prime example of this system, with the Democratic and Republican parties being the two dominant parties. In contrast to systems like proportional representation, the winner-take-all electoral system in the U.S. leads to this two-party dominance because it tends to exclude third parties from winning any national offices, as they need a plurality of votes in elections to win a seat. This system is deeply rooted in U.S. history and is reinforced by the election rules, which includes winner-take-all elections and challenges faced by third parties in gaining ballot access and voter support.