Which Amendment reserves powers to the states?

The Tenth Amendment of the United States Constitution specifically reserves powers not delegated to the federal government to the states and the people. This amendment is part of the Bill of Rights and was ratified in 1791. Its purpose is to affirm the principle of federalism, which allows states to exercise authority over various matters within their own borders, as long as those powers are not explicitly given to the federal government by the Constitution.

This means that any power that the Constitution does not assign to the federal government, nor prohibit to the states, is retained by the states or the people. The Tenth Amendment is fundamental in delineating the balance of power between state and federal authorities, ensuring that states maintain a significant degree of autonomy.