The correct answer is D.
Although the RMS Lusitania was a British ship, when it was sunk by a German submarine in 1915, 234 American citizens died. Nevertheless, in 1916 the Sussex Pledge took place, being a promise made by Germany in which they claimed to attack with no previous warning any armed ship. However, in March 1916 Germany torpeded a French ship that had no weapons, meaning this, and announcing Germany the resimption of unrestricted submarine warfare. Finally, based on the German decission and having reasons because of the Lusitania deceased people, the US declares war on Germany in April 1917.