<em>B. The Zimmermann Telegram.</em>
Explanation:
The United States was neutral for the majority of the war and President Woodrow Wilson tried his best to not involve the country in World War I. When he ran for re-election many people associated him with the slogan "<em>he kept us out of war</em>."
When the Lusitania got sank by a German U-boat, numerous Americans were on board. This struck rage in the United States, some people even wanted to join the war because of this. There was also the continuing issue with Germany using unrestricted submarine warfare, but after the sinking of the Lusitania, they said they would stop doing this, which did not last long.
The Zimmermann Telegram and Germany continuing unrestricted submarine warfare is known to be the final straw for the United States, causing President Woodrow Wilson to finally ask Congress to declare war. The Zimmermann Telegram was a telegram that was going from Germany to Mexico but was intercepted by Great Britain. This telegram stated that if Mexico were to attack the United States, Germany would then give them land when they won the war.
