Final answer:
The U.S. joined World War I due to Unrestricted Submarine Warfare, the Zimmerman Telegram, and the Assassination of Archduke Franz Ferdinand. Germany's attacks on neutral ships and the secret proposal of alliance and land to Mexico outraged the American public and pushed the U.S. towards war. The correct option is B.
Step-by-step explanation:
The three events that led the United States to join World War I were Unrestricted Submarine Warfare, the Zimmerman Telegram, and the Assassination of Archduke Franz Ferdinand.
Unrestricted Submarine Warfare was a policy implemented by Germany, declaring that all ships, including those from neutral countries like the U.S., would be sunk without warning if they entered the blockade zone around Britain. This led to American ships being attacked and heightened tensions between the United States and Germany.
The Zimmerman Telegram was a secret communication from the German Foreign Minister Arthur Zimmermann to the German ambassador in Mexico. It proposed a military alliance between Germany and Mexico in the event of the U.S. entering the war. The offer included the return of territories to Mexico that it had lost to the United States. British intelligence intercepted and decoded this message, and its revelation to the American public generated outrage.
The Assassination of Archduke Franz Ferdinand triggered a series of events that led to the outbreak of World War I. While this event did not immediately bring the U.S. into the conflict, it set the stage for the ensuing alliance confrontations which ultimately involved the United States.