C. It would prevent high casualties that would be caused by an invasion of mainland Japan
Explanation:
Although there are more reasons, out of the answers provided, c is the best answer. An invasion of mainland Japan would cost many American lives, which the US wanted to avoid.
Things to keep in mind:
- Germany already surrendered by the time the bombs were dropped
- Great Britain and the US were (and still are) allies
- There were almost 200,000 casualties from the two bombs