The correct answer is D) the battle of Okinawa.
The battle or event that caused the most Japanese casualties was the battle of Okinawa.
During World War II, the Battle of Okinawa was one of the bloodiest battles in the conflict. The United States Marine Corps invaded the Ryukyu Islands and Okinawa on April 1, 1945, as part of Operation Iceberg. After destroying the Japanese Army in Iwo Jima, the U.S. Army needed to occupy Okinawa before getting into Japan. It is believed that 110,000 Japanese soldiers and 100,000 civilians were killed.