Hello!
Before WWII, the United States favored isolationism. This meant that the country made an effort to avoid foreign conflicts that were not directly related to the U.S.
The United States also planned to avoid involvement in WWII. However, the Japanese bombed Pearl Harbor without declaring war on the U.S., which soon brought the U.S. into the war.
The correct answer to the question you have asked is A: The bombing of Pearl Harbor.
I hope this helps you answer your question.