Which of the following powers is explicitly denied to the states by the Constitution?

The power to enter into treaties with foreign governments is explicitly denied to the states by the Constitution. This prohibition ensures that foreign relationships and negotiations are managed exclusively by the federal government, maintaining a unified approach to diplomacy and international affairs. Article I, Section 10 of the Constitution clearly states that no state shall enter into any treaty, alliance, or confederation. This is crucial for maintaining national coherence and preventing individual states from acting independently in matters that could complicate international relationships or create conflicts with the federal government's foreign policies.

In contrast, the other powers listed have not been constitutionally denied to the states. While states may impose their own taxes (though they must adhere to certain limitations), regulate commerce within their own borders, and maintain their own militias, these actions are subject to certain federal regulations and limitations as established by the Constitution. However, the ability to enter into treaties is specifically reserved for the national government, highlighting the importance of a cohesive and singular diplomatic front.