Boris Yeltsin played a crucial role in the disintegration of the Soviet Union, particularly during the August 1991 coup attempt against Gorbachev. Yeltsin was the President of the Russian Soviet Federative Socialist Republic (RSFSR) and emerged as a key figure opposing the hardline communist coup. His defiance and leadership during the crisis marked him as an important figure in the eventual collapse of the Soviet Union.
Thus, the correct answer is (A) Boris Yeltsin.