Final answer:
The political party system in the United States since the 1960s is characterized by parity between the Republican and Democratic parties.
Step-by-step explanation:
The best characterization of the political party system since the 1960s in the United States is c. parity between the Republican and Democratic parties.
The United States has maintained a two-party system, with the Republican and Democratic parties being the dominant players. While other minor or third parties exist, the winner-take-all Electoral College system and state ballot rules make it difficult for them to be successful on a national level. Therefore, the two major parties have had a relatively equal presence in the political landscape since the 1960s.