Answer: Popular Sovereignty
Explanation: In the passage it stated that “ The [Kansas-Nebraska Act] provided that the people of the territory should themselves determine whether it should be slave or free”. Popular Sovereignty is the principle that authority of a state and government are created and sustained by the consent of its people. So basically it is saying that the people have a right to voice their opinion on what should happen in the state of government, in this case determining if Kansas should be a free or slave state.