Answer:
D. German submarine warfare
Step-by-step explanation:
The Unrestricted submarine warfare was effecting the lives of Americans traveling across seas, one example is the sinking of the RMS Lusitania. Which some of the passangers were Americans. The Zimmerman telegram was also a factor in the US entering WW1 but did not effect the individual lives of Americans. The Treaty of Versailles is not in place yet and British propoganda was also a factor in the US siding with France and Britain.