Step 1: Principal organs of the United Nations
The United Nations (UN) has six principal organs:
1. General Assembly
2. Security Council
3. International Court of Justice (ICJ)
4. Economic and Social Council (ECOSOC)
5. Trusteeship Council (currently inactive)
6. Secretariat
Step 2: Analyzing the options
- (A) Security Council: One of the six principal organs.
- (B) General Assembly: Also a principal organ.
- (C) Secretariat: A principal organ, headed by the Secretary-General.
- (D) World Bank: Not a principal organ of the UN. It is a specialized international financial institution that works independently, though in partnership with the UN.
Step 3: Conclusion
Thus, the correct answer is World Bank, as it is not a principal organ of the United Nations.
\[
\boxed{\text{Correct Answer: (D) World Bank}}
\]