Where is the headquarters of the ’Organisation of the Petroleum Exporting Countries’ (OPEC)?

The correct option is Vienna.

The Organisation of the Petroleum Exporting Countries (OPEC) is a permanent, intergovernmental organization created at the Baghdad Conference in September 1960 by its founding members: Iran, Iraq, Kuwait, Saudi Arabia, and Venezuela.

For the first five years of its existence, the organization's headquarters were located in Geneva, Switzerland. However, the OPEC Conference decided to relocate the headquarters, and on September 1, 1965, they were officially moved to Vienna, Austria, where they have remained ever since.