The correct is B) the rise of the Free-Soil Party.
In the election of 1848, the major parties were forced to openly state their position on slavery because of the rise of the Republican Party.
The Free-Soil Party only had one dominant issue: free slaves. They wanted to stop the expansion of slavery into other states. They promoted "free man for free soil" idea because it was morally superior for a country like the United States. The party that originated in New York was active in the presidential elections from 1848 to 1852. With the presence of the Soil Party, the other political parties had to openly declare their position on the issue of slavery.
The other options of the question were a) the rise of the American Party, c) the rise of the Republican Party, and d) the rise of the Whig Party.