Question

What new weapon made allied leaders worry about the cost of invading the Japanese mainland

Answer 1

The correct answer is Kamikaze fighters, I just took the quiz. Notice it asks about the Allied leaders being worried, not the Axis leaders. The Allies had the atomic bomb, and the Kamikaze fighters attacks were becoming more common when the US was approaching the mainland. Kamikaze attacks started in 1944.

Answer 2

The newfound Hydrogen Bombs and Atom Bombs.