Final answer:
The correct word to complete the principle highlighted in the Tenth Amendment is 'reserved,' which refers to the powers that are held by the states or the people, according to the U.S. Constitution's establishment of federalism.
Step-by-step explanation:
The word that best completes the principle of United States government mentioned in the Tenth Amendment to the United States Constitution is reserved. The Tenth Amendment states: "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people." This amendment is fundamental in establishing the concept of federalism within the U.S. government, where certain powers are allocated to the state governments, known specifically as reserved powers. These are powers not explicitly listed in the Constitution as belonging to the federal government and are not forbidden to the states, thus they are held by the states or the people.
The concept of reserved powers traces back to the debates between the Federalists and Anti-Federalists. The inclusion of the Tenth Amendment in the Bill of Rights was largely influenced by the Anti-Federalists who were concerned with maintaining state sovereignty and ensuring that the federal government would not overstep its boundaries.