The correct answer is A) the Battle of Iwo Jima.
What enabled President Truman and Allied leaders to issue the ultimatum above was the Battle of Iwo Jima.
The ultimatum was issued as part of the Postdam Conference of July 26, 1945, when the Allied forces demanded the government of Japan to surrender and put an end to World War II. This important document was signed by British Prime Minister Winston Churchill, the Chairman of China Chiang Kai Shek, and US President Harry S. Truman. Japan did not surrender and President Truman decided to launch the atomic bomb over Hiroshima and Nagasaki on August 6 and 9, respectively.