Answer : 4. "Qatar"
Explanation :

On December 21, 2023, Angola announced that it was leaving the Organization of the Petroleum Exporting Countries (OPEC) after 16 years of membership. With this move the number of OPEC members will reduce to 12. OPEC was established in 1960 by five founding members Iran, Iraq, Kuwait, Saudi Arabia and Venezuela. Currently, it has 13 members, including Algeria, Angola, Congo, Equatorial Guinea, Gabon, Libya, Nigeria and the United Arab Emirates. It is headquartered in Vienna, Austria. Qatar ended its membership in 2019. Qatar was once a member of OPEC but decided to leave the organization. The country announced its decision to withdraw from OPEC with effect from January 2019. The decision was made to focus more on its natural gas production, for which Qatar is one of the world's leading producers.