The most fundamental difference between the United Nations and the League of Nations was

The correct answer is D. effectiveness.

The most fundamental difference between the United Nations (UN) and the League of Nations lies in their effectiveness in maintaining international peace and security. The League, established after World War I, struggled to enforce its resolutions and ultimately failed to prevent World War II. In contrast, the UN, founded after World War II, has been more effective in addressing conflicts, thanks to its broader membership, more structured framework, and the ability to authorize peacekeeping missions.