The statement that describes a major reason that America entered World War I on the Allied side is:
Germany made a secret offer to Mexico to form an alliance to fight the United States.
This refers to the Zimmermann Telegram, in which Germany proposed a military alliance to Mexico if the U.S. entered the war against Germany. This revelation, along with other factors such as unrestricted submarine warfare by Germany and economic ties to the Allies, played a significant role in influencing the U.S. decision to join the war.