Step 1: Understanding the Question:
The question asks to pick the option that is not one of the main structural bodies of the United Nations.
Step 2: Key Fact or Approach:
The United Nations Charter sets up six principal organs: the General Assembly, the Security Council, the Economic and Social Council, the Trusteeship Council, the International Court of Justice, and the Secretariat.
Step 3: Detailed Explanation:
The General Assembly is where all member states meet and vote, so it is a principal organ, ruling out option (A).
The Trusteeship Council was set up to oversee trust territories moving towards self government, so it is also a principal organ, ruling out option (B).
The Security Council, responsible for maintaining international peace and security, is likewise one of the six principal organs, ruling out option (C).
The World Health Organization, in contrast, is a specialised agency that works under the UN system on global health matters, but it is not one of the six bodies created directly by the UN Charter as a principal organ.
Step 4: Final Answer:
The World Health Organization is not a principal organ of the UNO, so option (D) is correct.