The free-soil party’s platform could best be described as
(a) opposing the expansion of slavery into western territories
(b) calling for the expansion of the homestead act, providing cheaper
land sales in the west
(c) calling for the restriction of immigrants coming into the united
states
(d) advocating the notion of popular sovereignty in western territories
(e) opposing protective tariff s and high domestic excise taxes

The Free-Soil Party’s platform could best be described as "(A) opposing the expansion of slavery into western territories," since the question of whether or not newly-admitted states in the US would be "slave" or "free" was that main issue that led to the Civil War. 

Leading up to the Civil War, the event that occurred first was D. the passage of the Compromise of 1850.

It is called the Compromise of 1850 to the passage of five bills by the United State Congress in 1850. Moreover, these laws, which dealt with the issue of slavery, were proposed by Henry Clay of Kentucky, who sought to end the conflict between the North and the South. After the bills were passed, the slave trade in Washington was abolished. The passage of the Compromise of 1850 occurred before the presidential election of Abraham Lincoln in 1860, John Brown's raid on Harpers Ferry in 1859 and the Dred Scott decision in 1857.