The Soviet Union, a socialist state, existed from 1922 until its dissolution in 1991, marking a pivotal moment in global history and the end of the Cold War.
The correct answer is Mikhail Gorbachev. He served as the last General Secretary of the Communist Party of the Soviet Union from 1985 and later as the first and last President of the Soviet Union from 1990 until its dissolution in December 1991. His policies of Glasnost (openness) and Perestroika (restructuring) ultimately led to the collapse of the USSR.
Boris Yeltsin is incorrect; he became the first President of the Russian Federation *after* the Soviet Union's collapse, playing a key role in its dissolution but not leading the USSR itself. Leonid Brezhnev is incorrect; he was a long-serving Soviet leader from 1964 to 1982, preceding Gorbachev by several years.