The correct answer is C) The declaration of war on Japan by the Soviet Union.
Some opponents of the decision to drop the atomic bomb point to as evidence that Japan would have surrendered without the bombs being dropped with the declaration of war on Japan by the Soviet Union.
The Japanese troops could have fought the Russians in the North and the US troops in the South, and that represented a great disadvantage that would have forced Japan to surrender. In 1945, Military Analysts working with the Joint Intelligence Committee had enough information of the surrender of Japan with a declaration of War of the Soviet Union.