The response that BEST describes the events leading up to the United States' involvement in World War I is:
D: American citizens wanted to stay neutral unless innocent Americans were killed.
This reflects the general sentiment in the U.S. at the beginning of the war, where many people favored neutrality and were reluctant to engage in European conflicts unless American lives were directly threatened, which ultimately did occur with incidents such as the sinking of the Lusitania and the Zimmerman Telegram.