Final answer:
The United Nations has the most member countries out of the options provided, with a total of 193 member states and 2 observer states.
Step-by-step explanation:
The organization with the most member countries is the United Nations. The United Nations currently has 193 member states and 2 observer states, which makes it the most universal and most recognized international organization. On the other hand, the North Atlantic Treaty Organization (NATO) is a military alliance with 30 member states, and the Organization of the Petroleum Exporting Countries (OPEC) has 13 member states. The United Nations' purpose includes maintaining international peace and security, developing friendly relations among nations, promoting social progress, and better living standards and human rights.