The correct answers are B) widened debate in the campaigns and D) shone a light on issues important to them.
The most significant effects of third parties in the US such as the Free Soil Party were the following: they widened debate in the campaigns and shone a light on issues important to them.
Although third parties do not represent a real threat to the bipartisan system in the United States dominated by the Republican Party and the Democratic Party, they widened debate in the campaigns and shone a light on issues important to them.
Since the People's Party, organized by populists in the early 1900s or different parties that have participated in elections such as the Reform Party that nominated Ross Perot in 1992, third parties make American citizens understand new perspectives on issues that have to bear in mind.