The kansas-nebraska act nullified the missouri compromise by
a. allowing slavery's spread to areas that had been free for more than 30 years. b. ending slavery in agreed-upon slave territories.
c. allowing slavery in all states.
d. abolishing slavery in slave states.

B :)
The Kansas-Nebraska Act of 1854 allowed settlers to decide whether their state will allow slavery or not.

The Missouri Compromise of 1820 was done to maintain the same number of free-states and slave-states. Missouri wanted to become a part of the slave-state but its admission with create an imbalance to the proportion of free-states and slave-states. They decided under the compromise, that Missouri will be a slave-state while Maine (former part of Massachusetts) will be a free state. Bring the number of slave and free states at an equal 12 states each. 

Answer for apex - the reformation