Popular Sovereignty was the idea that

The people of a territory should determine for themselves whether or not to permit slavery, is the right answer.

Explanation:

Popular sovereignty is the belief that the power of a state and its management are designed and maintained by the acquiescence of its people, by their elected delegates, who are the roots of all legislative authority. It is firmly connected with social convention scholars such as Jean-Jacques Rousseau, John Locke and Thomas Hobbes. Popular sovereignty denotes a theory and does not fundamentally follow or express a legislative reality. In the context of Slavery, Popular Sovereignty was the notion that people of a colony should decide for themselves whether or not to authorize slavery.

The idea and concept of Popular Sovereignty is the belief that the authority, legality and legitimacy of the government is created by the will or consent of its people.