Which of the following is not a member of 'Gulf Cooperation Council'?

Iran is not a member of the Gulf Cooperation Council (GCC) because the organization is a political and economic union consisting exclusively of six Arab monarchies: Saudi Arabia, Kuwait, the United Arab Emirates, Qatar, Bahrain, and Oman. While Oman might be a tempting wrong option due to its historically neutral foreign policy compared to its neighbors, it is a founding member of the bloc. This question tests the core concept of regional geopolitical alliances and the specific membership of key international organizations in the Middle East.