The correct answer to this open question is the following.
Although there are no options attached we can say the following.
The reason based on the events of the last decade of the Cold War, that historians might still debate the respective influences of Reagan and Gorbachev on the end of the Cold War could be the good relationship both leaders had that led to a political and professional solution to control exerted by the Soviet Union in Eastern Europe. We could even say that both -Reagan and Gorbachev- kind of developed a friendship that was conducive to the end of the Cold War between the United States and the Soviet Union.
The world witnessed how these two leaders agreed on many political and economic issues to the degree they both accepted the necessity of overcome obstacles and differences and reinitiating the reduction of nuclear weapons negotiation.