Disintegration of the Soviet Union

Khrushchev

After Stalin's death in 1953, Khrushchev took power and began the de-Stalinisation campaign. He publicly condemned the wrong actions of Stalin, purged Stalin's loyal comrades, and restored relations with Tito of Yugoslavia. Brezhnev followed Khrushchev and continued authoritarian rule. Andropov came after Brezhnev. Gorbachev came much later in 1985 with his own set of reforms.

Doctrine of limited sovereignty

The Brezhnev Doctrine, also called the 'doctrine of limited sovereignty', stated that a member of the Warsaw Treaty had no right to take a step unacceptable to the Soviet leadership. This was used to justify Soviet intervention in Czechoslovakia in 1968. Self-determination was a principle the Soviets claimed to support but rarely allowed. Detente was the policy of relaxation of tensions between the superpowers. Non-alignment was a movement of countries not joining either bloc.

Gorbachev

Mikhail Gorbachev came to power in March 1985 and introduced landmark reforms like Glasnost and Perestroika. He realised that the Soviet system had failed from within and introduced 'New Thinking' in both domestic and foreign policy. Andropov came to power in 1982 but died after about a year. Brezhnev was in power from 1964 to 1982. Chernenko came after Andropov and also had a short term (1983-85).

USSR, Poland, Czechoslovakia, Hungary and East Germany among others

The Warsaw Treaty (1955) was signed by USSR, Poland, Czechoslovakia, Bulgaria, Romania, Hungary, Albania, and East Germany. It formed the socialist military bloc led by the Soviet Union. The USA, UK, France, and West Germany were part of NATO, the Western military alliance. India, China, Egypt, and Yugoslavia were not part of the Warsaw Pact. NATO (Option D) was the Western alliance, not the Warsaw Pact.