Answer: 1991
Explanation: It was dissolved after the failed attempt of communist hardliners to overthrow Gorbatchev from the leading position in the country. In 1991 (March) there was a referendum (three republics were against) where the people of USSR decided to make a new federation. In late 1991 there was a failed coup against Gorbatchev after which Gorbatchev resigned. After this resignation USSR was formally dissolved by the Supreme Soviet.