The United Nation Organisation (UNO) is primary regarded as successful because

Step 1: Analyze the question The question asks for the main reason the UN is regarded as successful. The UN was founded in 1945, after World War II, with the primary aim of maintaining international peace and security.

Step 2: Explain the concept The UN's central purpose, set out in Article 1 of its Charter, is to prevent war and keep peace between nations. It does this through the Security Council, peacekeeping missions, diplomacy, sanctions and mediation. So its success is judged mainly by how well it has stopped another global conflict like the two World Wars.

Step 3: Evaluate each option

• A. Its members continue to increase. The UN grew from 51 founding members to 193. This shows wide acceptance, but growth in membership is not the main measure of success. It is not the primary reason.
• B. Of its control of outbreak of another world war. This matches the UN's core mission. Since 1945 there has been no third world war, and the UN's role in diplomacy, conflict resolution and peacekeeping is widely credited as part of the reason. This is the best answer.
• C. All member countries have veto power. This is false. Only the five permanent members of the Security Council (USA, UK, France, Russia, China) have veto power.
• D. More member countries acquire more nuclear power. This is not a success. The UN works to limit nuclear proliferation, not encourage it.

Step 4: Answer

Exam tip: For questions on why an international organisation is "successful", go back to its founding purpose. For the UN it is peace and security, so choose the option that reflects preventing war. Also remember that veto power belongs only to the P5, and watch for options that say "all members".