Why would countries agree to join such an organization?

Because the supranational organization offers a way to set international rules governing the affairs of nations, with an eye to preventing conflicts. ... Supranational organizations often give member states greater collective influence in global affairs.

Explanation:

The basic idea is that a national government gives up some of its powers to an international organization. Nationalism, however, remains an important obstacle to the growth of supranationalism. ... Supporters of supranationalism believe the world's problems can best be addressed by groups of cooperating nations.

answer; i believe the correct answer is prefrontal cortex;

the answer your looking for is c. : )