Answer:
d. Soviet Union dissolved, ending the Cold War
Explanation:
The head of the USSR at the moment Mikael Gorbachov decided that the time had come for a series of Reforms and major changes in the structures of the economic and political bodies.
By approaching the West he formalized the dissolution of the Soviet Union. The Russian and Americans began to seek cooperation in a series of topics that would pose a new agenda.
No longer could Cold war politics prevail.
Especially, the threat of terrorism that both sides suffered, made strong efforts into defining a new policy sometimes reflected in bilateral agreements.
With the collapse of the Soviet Union started in the late 1980 countries in the periphery of Russia began to approach US bilateral cooperation as well.
The resulting 15 independent states after 1991 meant also an effort to demilitarize the region. The case was most clear in Ukraine that gave up all the nuclear warheads to Russia.
This timeline shows the leaders of the Soviet Union and when they ruled starting with Kerensky, the head of the Provisional Government, and ending with Gorbachev in 1991 when the Russian Republic was formed. http://www.rff.com/timeline_soviet.htm
