Answer:
A. Dropped two atomic bombs on Japan.
Explanation:
after the U.S. dropped two atomic bombs on Japan, the Japanese country surrendered and lost in WWII.
why other answers we're wrong:
B. the assassination of Adolf Hitler is wrong because Hitler killed himself after Japan surrendered.
C. Fought the Japanese at pearl harbor is wrong because it happened before WWII.
D. send a million-man army to France is wrong because the U.S. dropped off 5 million soldiers in France at the start of WWII.