The correct answer is option 'D' - International Monetary Fund (IMF) is not the main organ of the United Nations.

The United Nations (UN) is an international organization founded on October 24, 1945, with the aim of promoting peace, security, and cooperation among member states. It consists of several main organs that work together to achieve these goals. Let's explore each of these organs and understand why the International Monetary Fund is not one of them.

1. General Assembly:
The General Assembly is the main deliberative and policy-making organ of the UN. It is composed of all 193 member states and provides a forum for discussions on global issues, international cooperation, and the adoption of resolutions. It meets annually and has the power to make recommendations to member states and coordinate international action.

2. Security Council:
The Security Council is responsible for maintaining international peace and security. It consists of 15 member states, including five permanent members (China, France, Russia, the United Kingdom, and the United States) with veto powers. The Security Council can impose sanctions, authorize military actions, and mediate in conflicts.

3. Economic and Social Council (ECOSOC):
The Economic and Social Council coordinates international economic, social, and environmental issues. It consists of 54 member states elected by the General Assembly for a three-year term. ECOSOC promotes sustainable development, human rights, and international cooperation in various areas such as education, health, and the environment.

4. Secretariat:
The Secretariat is the administrative arm of the United Nations. It is headed by the Secretary-General, who is appointed by the General Assembly upon the recommendation of the Security Council. The Secretariat provides support to the other organs, implements their decisions, and coordinates the work of UN agencies, programs, and funds.

5. International Court of Justice (ICJ):
The International Court of Justice is the principal judicial organ of the United Nations. It settles legal disputes between member states and provides advisory opinions on legal questions referred to it by authorized UN organs or specialized agencies. The ICJ consists of 15 judges elected by the General Assembly and the Security Council.

6. International Monetary Fund (IMF):
While the IMF is an important international financial institution, it is not one of the main organs of the United Nations. The IMF is a separate entity established in 1944 to promote global monetary cooperation, secure financial stability, facilitate international trade, and foster economic growth. It works with member countries to provide financial assistance, policy advice, and technical assistance in areas related to macroeconomic stability and economic development.

In conclusion, the International Monetary Fund (IMF) is not a main organ of the United Nations. The main organs of the UN are the General Assembly, Security Council, Economic and Social Council, Secretariat, and International Court of Justice. Each organ has specific roles and responsibilities in achieving the objectives of the United Nations.