B. The Zimmerman Telegram
Step-by-step explanation:
The United States finally joined World War I after the Zimmerman Telegram. It did take a while, as President Woodrow Wilson was a pacifist and wanted to avoid the United States going into a war as much as possible.
The sinking of the Lusitania was also a reason the United States declared war on Germany. This was a ship that got sunk by German ships, that had Americans on board. This angered many Americans, but Woodrow Wilson still avoided joining the war.
The Zimmerman Telegram was the final straw. This was a telegram that got intercepted by the British, that was from Germany to Mexico. Germany stated that if Mexico sided with them and attacked the United States, they would get territories when Germany won the war.