The principle of Federalism would be the correct answer. Article Five of the US Constitution states that for an amendment to be made to the Constitution, it has to be proposed by either a convention of states or by passing with two thirds of the votes through both the House of Representatives and the Senate.
Because Congress is composed of elected representatives from all the different states, while the Constitution is the legal frame work for the federal government, this means that such process reflect the principle that the states that make up the Union are responsible for modifying the federal Constitution which is an instance of Federalism.
