Answer: Popular Sovereignty
Step-by-step explanation:
Prior to the Kansas-Nebraska Act, the slave status of a new territory would be decided by the Missouri Compromise which based the state's slave status on geographical location as it prohibited slavery in states to the North of the 36°30′ parallel (excluding Missouri).
In 1854 however, a bill that would later be known as the Kansas-Nebraska Act was introduced to Congress by Sen. Stephen A. Douglas who hoped to gain support from Southern politicians for a state to be established on land gained from the Louisiana purchase.
The bill called for the status of a state to be decided by Popular Sovereignty which essentially meant that the people of the state would decide whether or not they wanted to be a free state instead of Congress as had previously been the case.
With this act therefore, the new territories would decide their status by themselves.