Concept:
The United Nations Conference on Trade and Development (UNCTAD) is a permanent intergovernmental body of the United Nations, established in 1964 to support developing countries in trade, investment and development issues.
Explanation:
UNCTAD's secretariat is headquartered in Geneva, Switzerland, the same city that hosts several other UN bodies such as the WHO and the ILO. Option A, Vienna, hosts agencies such as the IAEA and UNIDO, not UNCTAD. Option C, Rome, is the headquarters of the FAO and WFP. Option D, Paris, hosts UNESCO. Since UNCTAD's own headquarters is specifically in Geneva, the other cities can be ruled out.
Final Answer:
UNCTAD is located in Geneva, option B.