In this scenario, the best power for the U.S. to use to influence Country A to get rid of its nuclear weapons would likely be:
D) economic.
Given that Country A is a poor country with little food, leveraging economic tools such as sanctions relief, aid, or incentives for disarmament could potentially persuade the leadership to reconsider their nuclear ambitions. Economic engagement could be framed as a pathway to improve the welfare of its people, which may be a compelling argument for the government, especially if they are aware of their population's struggles.
While diplomatic efforts may also be important, they could be less effective given the closed nature of the country and its perception of the U.S. as an enemy. Military options could escalate tensions and provoke a hostile reaction. Informational efforts might be challenging due to limited access to outside media. Therefore, economic influence is the most viable option to offer incentives for disarmament while potentially improving conditions for the citizens of Country A.