The correct answer is D.
On August 6 1945, the U.S. dropped the first atomic bomb on Hiroshima and three days later, a second one on Nagasaki. On August 15 1945, the surrender of the Empire of Japan was announced by Imperial Japan and was officially signed on September 2 1945 bringing the second world war to a close.
The Soviets had declared war on Japan on the 8th of August just before Nagasaki was bombed. This plus the dropping of the atomic bombs put Japan in a hopeless position to continue fighting and she was thus forced to surrender, and that unconditionally.