The correct answer is: D. The General Assembly.
The General Assembly is the main deliberative organ of the United Nations. It is composed of all 193 Member States of the United Nations. The General Assembly meets in regular sessions every year, and in special sessions when necessary.
The General Assembly has the power to discuss and make recommendations on any matter within the scope of the Charter of the United Nations. It also has the power to initiate studies and make recommendations to promote international cooperation in the political, economic, social, cultural, educational, and health fields, and to further international peace and security.
The General Assembly is the only organ of the United Nations in which all Member States have equal representation. This makes it the most representative organ of the United Nations, and the one that most fully represents the fact that the United Nations is a universal organization.
The Economic and Social Council (ECOSOC) is a functional organ of the United Nations. It is responsible for promoting international economic and social cooperation and development. ECOSOC is composed of 54 Member States, elected by the General Assembly for three-year terms.
The Security Council is a principal organ of the United Nations. It is responsible for maintaining international peace and security. The Security Council has 15 members, including five permanent members (China, France, Russia, the United Kingdom, and the United States) and 10 non-permanent members elected by the General Assembly for two-year terms.
The Secretariat is the administrative organ 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 is responsible for carrying out the decisions of the General Assembly, the Security Council, and other organs of the United Nations.