Which amendment of the U.S. Constitution reserves powers to the states?

The Tenth Amendment of the U.S. Constitution is pivotal because it explicitly reserves powers that are not granted to the federal government nor prohibited to the states, to the states themselves or to the people. This amendment is part of the Bill of Rights and serves to reinforce the principle of federalism by ensuring that states maintain a degree of sovereignty and authority over their internal affairs. This means that any powers that are not enumerated in the Constitution are left to the states to legislate and govern, which is crucial for the balance of power between state and federal governments. In essence, the Tenth Amendment acts as a check on federal authority and a protector of state's rights, establishing a key aspect of the constitutional framework of the United States.

The other amendments listed do not serve the same purpose. The First Amendment addresses rights related to free speech, religion, and assembly; the Ninth Amendment deals with the enumeration of rights not specifically listed in the Constitution; and the Eleventh Amendment pertains to the jurisdiction of federal courts over state lawsuits. None of these focus on the reservation of powers to the states like the Tenth Amendment does.