It was Gorbachev because he was the former statesmen, served as a General Security of the Communist Party of the Soviet Union from 1985 until 1991, and as the first president of the Soviet Union from 1988 until its dissolution in 1991.

Union of Soviet Socialist Republics

the Soviet Union, or U.S.S.R, collapsed in 1991

It refers to the Soviet Union becoming 15 separate countries in 1991. What was the Soviet Union broke up into those countries. This was as a result of a lot of reforms and political upheaval in the Soviet Union which started when Mikhail Gorbachev became the president in 1985.

Supreme Soviet of the Soviet Union ended in 1991.