What major military alliance was formed in 1955 in response to NATO?

The major military alliance formed in 1955 in response to NATO is the Warsaw Pact. Established by the Soviet Union and its satellite states in Eastern Europe, the Warsaw Pact was created as a counterbalance to the North Atlantic Treaty Organization (NATO), which was seen as a Western military alliance primarily aimed at deterring Soviet expansion.

The Warsaw Pact solidified the military cooperation of its member states, which included countries like Poland, East Germany, Czechoslovakia, Hungary, Romania, Bulgaria, and Albania, along with the Soviet Union. This alliance facilitated coordinated military planning and collective defense among its members and was a pivotal aspect of the Cold War, illustrating the division between the Eastern and Western blocs.

The other options reflect either different international agreements or military alliances that were either not established specifically in response to NATO or did not exist in that specific context. For instance, while the SEATO Treaty and the Central Treaty Organization were multilateral defense agreements, they were focused on different regions and issues, primarily in Asia and the Middle East, rather than serving as a direct reply to NATO. The "Soviet Union Alliance" is a misleading term as it does not specifically refer to a formalized military coalition like the Warsaw Pact.