Which power is exclusively held by the states under the U.S. Constitution?

The power to regulate education is exclusively held by the states under the U.S. Constitution because the Constitution does not grant the federal government the authority to oversee education. Instead, education is considered a matter of state and local jurisdiction, allowing states to establish and manage their own education systems, curricula, and standards. This state control reflects the principle of federalism, wherein powers are divided between the national and state governments, with education being a key area traditionally managed at the state level.

In contrast, declaring war and negotiating treaties are powers that are reserved for the federal government, as outlined in the Constitution. These powers require a unified national approach and are essential for the conduct of foreign affairs and national defense. The maintenance of the postal service is also a federal responsibility, established to ensure a consistent and reliable communication system across the country, which states alone would struggle to provide effectively. Thus, education stands out as a function where states retain exclusive control.