what made allied leaders worry about the cost of invading the japanese mainland?\no kamikaze fighters\no…

Brief Explanations:

Kamikaze fighters were Japanese pilots who carried out suicide - attacks. Their willingness to die in combat would have made an invasion of the Japanese mainland extremely costly in terms of Allied lives. Machine guns, atomic bombs (used by the Allies), and warplanes were not what made the Allies worry about the cost of invasion in the context of this question.

Answer:

A. kamikaze fighters