Final answer:
Third parties in U.S. history have been most effective by highlighting issues ignored by the major parties, and while they rarely win major elections, they can influence outcomes by drawing votes from major parties.
Step-by-step explanation:
In the electoral history of the United States, third parties have been most effective as vehicles of protest when they dramatized issues and positions that were being ignored by the major parties (C). Although third parties have seldom been successful in winning national elections, they have served as 'safety valves' for voter discontent with the Republican and Democratic parties. They bring attention to specific issues and occasionally alter election outcomes by drawing votes from the major parties.
For instance, the Progressive Party in 1912 drew enough votes away from the Republicans to influence the outcome of the election in favor of the Democrats. Additionally, ideas championed by parties like the Populist Party have later been adopted by the major parties. Despite their impact, third parties often struggle to sustain success due to a lack of broad electorate support and competition with the major parties which can absorb their issues or supporters.