The correct answer is A) a splinter party.
Many of the most important minor parties have been splinter parties.
A splinter party is formed when members break away from one of the major parties. In this case, the major two parties in the United States are the Republican Party and the Democratic Party. When those members do not agree any more in ideological terms or set of beliefs with the major party, they leave and form their party. That is a splinter party, and many important minor parties have been splinter parties.
