Answer:
Section 4 of the U.S. Constitution is related to the guarantee of a republican form of government to each state in the Union and protection against invasion and domestic violence. The principle that best applies to this section is the principle of Federalism.
Federalism is the principle that divides power and authority between the federal government and the state governments. It establishes a system of government in which power is shared between a central government and constituent political units, such as states or provinces. Section 4 of the U.S. Constitution reflects the principle of federalism by recognizing the importance of state governments and their role in protecting their citizens and maintaining a republican form of government.
This principle ensures that states have some degree of autonomy and can make decisions that best suit their unique needs and circumstances, while also recognizing the need for a central government to provide for the common defense and general welfare of the nation as a whole. By guaranteeing a republican form of government to each state, the Constitution protects the rights of citizens and helps to ensure that each state is governed fairly and justly.