Question

The United States entered World War I on April 6, 1917, after:

the continued sinking of ships and loss of American life after the Lusitania
the stalemate of trench warfare was unresolved
the assassination of Austria's Archduke Ferdinand
the sinking of the Lusitania

Answer 1

the continued sinking of ships and loss of American life after the Lusitania

(Immediate causes in 1917 were Germany's resumption of unrestricted submarine warfare—leading to American ships/servicemen being sunk—and the Zimmermann Telegram; the Lusitania’s 1915 sinking helped sway opinion but was not the direct trigger.)