Final answer:
The restriction on states entering into a treaty on their own with another country is an example of federal power.
Step-by-step explanation:
The correct answer is d. Federal power. The restriction on states entering into a treaty on their own with another country is an example of federal power. According to Article | Section 10 of the U.S. Constitution, this power is given to the president, with the advice and consent of two-thirds of the Senate present. This provision ensures that foreign diplomacy and the power to negotiate treaties are centralized at the federal level, rather than being exercised by individual states.