The Zimmerman Telegram was just one more reason that the US got to enter World War I. It was supposed to be a secret piece of correspondence between Germany and Mexico, where Germany offered the military alliance to Mexico in case the US entered the War. The telegram was intercepted and decoded. This led to increase in public support for the US involvement in the War.
Answer: C) It was another reason for the US to join the war on the side of the Allies.
Further detail:
The US declaration of war to enter World War I was passed by Congress on April 6, 1917.
The reasons that led to declaring war:
In January, 1917, Germany had resumed its policy of unrestricted submarine warfare. Germany had halted its attacks on non-military vessels (which it suspected of carrying military supplies) after the furor over the sinking of the Lusitania in 1915. But now Germany was resuming attacks by its U-boats.
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 Woodrow 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.
