Correct Answer:
A. Tehran
The Economic Cooperation Organization (ECO) is a regional intergovernmental organization established in 1985 by Iran, Pakistan, and Turkey. Its primary goal is to promote economic cooperation among member states. The correct answer is Tehran because the ECO Secretariat, which serves as the organization's headquarters, is located in Tehran, the capital city of Iran. Istanbul is a major city in Turkey, Kabul is the capital of Afghanistan, and Karachi is a major city in Pakistan; while all these countries are ECO members, their cities do not host the main headquarters.