What is one power that belongs exclusively to the states under the Constitution?

The correct answer highlights one of the powers reserved for the states as outlined in the Constitution. States have the authority to provide protection through various means such as establishing and maintaining their own police forces, fire departments, and other emergency services, which are vital to ensuring the safety and welfare of their residents.

While providing protection is not the only power states have, it is indeed one that is fundamentally within their domain without encroaching on the federal government's responsibilities. The other choices represent powers that are primarily or exclusively held by the federal government. For example, declaring war and coining money are powers that belong to the federal government and are specifically mentioned in the Constitution as responsibilities that cannot be executed by the states. Regulating trade, while it can involve state powers in some local contexts, typically falls under the Commerce Clause, which grants the federal government the authority to regulate interstate and international trade. This clear delineation of powers is crucial for maintaining the balance of federalism within the United States.