The Central Asian republics, which include Kazakhstan, Uzbekistan, Turkmenistan, Kyrgyzstan, and Tajikistan, became founding members of the Commonwealth of Independent States (CIS).

Explanation:
- The Commonwealth of Independent States (CIS) is a regional intergovernmental organization composed of former Soviet republics. It was established on December 8, 1991, after the dissolution of the Soviet Union.
- The main goals of the CIS are to promote economic and political cooperation, collective security, and cultural and humanitarian ties among its member states.
- The Central Asian republics, which were formerly part of the Soviet Union, joined the CIS as founding members in 1991. They were joined by other former Soviet republics such as Russia, Ukraine, Belarus, and others.
- Since then, the CIS has played an important role in the post-Soviet era, providing a platform for dialogue and cooperation among its member states.
- However, the effectiveness of the CIS has been questioned by some critics who argue that it has not been able to achieve its main goals and has been largely dominated by Russia.

In summary, the Central Asian republics became founding members of the Commonwealth of Independent States (CIS) in 1991, along with other former Soviet republics. The CIS has been an important regional organization for promoting cooperation and dialogue among its member states.