A. The Zimmerman Telegram --> the United States enters World War I.
Step-by-step explanation:
The Zimmermann Telegram was a note that was intercepted by Great Britain and is known for being one of the main causes of the United States joining World War I.
Great Britain had intercepted a note that was coming from Germany and going to Mexico. The note stated that if Mexico attacked the United States, Germany would give them land once they won the war. This obviously never reached Mexico because it was intercepted, but it still struck rage within the people of the United States.
President Woodrow Wilson was adamant about not joining World War I. He even won his re-election using the slogan, "he kept us out of war." Despite Germany sinking the Lusitania, which was an ocean liner with Americans on-board, or them continuing to use unrestricted submarine warfare, he still tried his best to keep the United States out of World War I.
This could not be helped any longer though with the Zimmermann Telegram and the continuing unrestricted submarine warfare coming from Germany. President Woodrow Wilson then asked Congress to declare war and on April 6, 1917, the United States had officially joined World War I.