The correct answer is A. OPEC.
A cartel is a formal agreement among competing firms in an industry to fix prices, limit output, or share markets. OPEC is the Organization of the Petroleum Exporting Countries, a cartel of 13 oil-producing countries. OPEC was founded in 1960 to coordinate and unify the petroleum policies of its member countries and to secure fair and stable prices for petroleum producers. OPEC has been successful in raising oil prices and in controlling the global oil market.
The other options are not examples of cartels. The International Monetary Fund (IMF) is an international financial institution that provides loans to countries experiencing balance-of-payments difficulties. The World Trade Organization (WTO) is an international organization that promotes trade between countries. The United Nations (UNO) is an international organization that promotes international co-operation and peace.