Which of the following is an example of a power reserved for the states?

The seventh amendment to the Constitution, along with the Tenth Amendment, provides a framework for understanding the powers reserved for the states. Establishing public schools falls clearly within this framework, as education is traditionally managed at the state and local levels. This is grounded in the principle of federalism, where states are afforded the authority to govern within their own territories and make decisions on issues such as education, health, and welfare.

The other options relate to powers that are primarily reserved for the federal government. Regulating foreign trade, conducting foreign affairs, and establishing a postal system are all actions that fall under the jurisdiction of the national government. This division of powers ensures that while states have significant authority in certain areas, specific responsibilities are assigned to the federal government to maintain a cohesive national strategy. Therefore, establishing public schools exemplifies the states' reserved powers effectively, emphasizing the role of state governments in managing local educational systems.