What decision forced Japan to surrender and quickly end World War II?

Question

Responses
1. to invade the Japanese mainland
2. to run 24-hour bombing raids against the Japanese capital city of Tokyo
3. to engage in unrestricted submarine warfare
4. to use nuclear weapons on the military targets in Hiroshima and Nagasaki

Answer 1

4. to use nuclear weapons on the military targets in Hiroshima and Nagasaki