In 1860, the Democratic Party split into two parts over ________.
In 1860, the Democratic Party split into two parts over national slave codes, which called for all states to have slavery. In 1860, the Southern Breckinridge Democrats introduced a plank into the Democratic Party platform calling for a national slave code: Slavery would be legal everywhere. The Northern Douglas Democrats refused to pass it. The Democrats tried to reunite but failed. That was the origin of the two Democratic Parties in the 1860 election.
Each of the other options refers to earlier controversies.
a,b. California was admitted as a Free State in 1850, and Missouri in 1820.
d. The issue of internal improvements was a major controversy in the 1830s, and it was the Republicans who called for a transcontinental railroad in the 1860 election.
e. The bank (E) was a key issue in the 1830s.