Step 1: Fact recall.
The World Trade Organization (WTO) was established in 1995 and its permanent secretariat is based in Geneva, Switzerland.
Step 2: Analyze options.
(A) New York: UN HQ is here, not WTO.
(B) Geneva: Correct — WTO headquarters.
(C) Hanoi: Not related to WTO HQ.
(D) Paris: OECD/UNESCO HQs here, not WTO.
Step 3: Conclusion.
Hence, WTO HQ is in Geneva.