Since you didn't list choices, I can just provide you a summary of what led to the United States' involvement in World War I.
Public outrage in the US against the Germans swept the nation following the sinking of the British ocean liner, Lusitania. When a German U-boat (submarine) sank the Lusitania in May, 1915, over 1,000 persons were killed, including more than 100 Americans. The passenger liner was targeted by the Germans because they suspected weapons were being shipped to Britain in the cargo hold of the ship. Germany managed to stave off American entry into the war at the time by pledging to stop submarine attacks.
In January, 1917, Germany resumed its policy of unrestricted submarine warfare, again attacking non-military vessels which they suspected of carrying military supplies. The resumption of Germany's U-boat warfare again inflamed tensions with the United States.
In February, 1917, the "Zimmerman Telegram" was intercepted by British intelligence and shared with the US. Germany's foreign minister, Arthur Zimmerman, had telegraphed an offer to Mexico's ambassador seeking Mexico's support in war vs. the United States in exchange for getting land back from the US.
On April 2, 1917, President Wilson made a powerful speech to Congress in which he argued that the nation needed to enter the war "to make the world safe for democracy." Wilson's speech was powerfully convincing, and four days later, Congress declared war.
