what made allied leaders worry about the cost of invading the japanese mainland?\nkamikaze fighters\nmachine…

Brief Explanations:

Kamikaze fighters were Japanese suicide - attack aircraft. They were expected to cause significant losses to Allied forces during an invasion of the Japanese mainland, thus worrying Allied leaders about the cost. Machine guns, atomic bombs, and warplanes were not the main factors causing such worry in this context.

Answer:

A. kamikaze fighters