Which of the following powers is forbidden to the states?

The power to coin money is explicitly forbidden to the states by the United States Constitution. The Constitution grants the federal government the exclusive authority to mint currency. This provision ensures a uniform national currency and helps maintain the integrity of the nation's monetary system. Allowing individual states to create their own currency could lead to economic chaos, such as fluctuations in value and complications in trade between states. By concentrating this power at the federal level, the Constitution seeks to prevent such instability and promote a cohesive economic policy across the nation.

In contrast, states have the authority to establish schools, regulate commerce within their borders, and impose income taxes, as these powers are not reserved exclusively for the federal government in the Constitution. This differentiates the power to coin money as a unique restriction specifically placed on state powers.