Question

in 1945, what concern did the allies have about invading the japanese mainland with ground forces?
they were worried they did not have enough soldiers.
they were worried there would be too great a loss of life.
they were worried the japanese were developing an atomic bomb.
they were worried the japanese had a stronger fleet in mainland ports.

Explanation:

In 1945, the Allies were concerned about a ground - force invasion of the Japanese mainland due to the potential for extremely high casualties. The Japanese had shown fierce resistance in previous battles (such as the Battle of Okinawa), and estimates suggested that a mainland invasion could result in hundreds of thousands or even millions of Allied and Japanese casualties.
The Allies had sufficient soldiers (they had large combined forces from multiple countries). The Japanese were not developing an atomic bomb (the US was the first to develop and use it). And the Japanese fleet had been severely damaged by 1945.

Answer:

They were worried there would be too great a loss of life.