Final answer:
The Zimmermann Telegram led to American support for entering WWI because it proposed a German-Mexican alliance against the US, promising Mexico the support to reclaim lost territories. This, coupled with German submarine warfare, incited American outrage and pushed the US towards war.
Step-by-step explanation:
The Zimmermann Telegram significantly influenced American public opinion and led to the United States entering World War I. The telegram was a secret communication sent by the German foreign minister, Arthur Zimmermann, to the German ambassador in Mexico. The message proposed a military alliance between Germany and Mexico if the United States entered the war on the side of the Allies. Germany promised to support Mexico in reclaiming lost territories such as Texas, New Mexico, and Arizona.
The British interception and decoding of this message, and its subsequent publication, caused outrage among Americans. Not only did the telegram suggest an alliance aimed against the United States, but it also exposed the threat of an invasion which could hinder U.S. support to the Allies. This, paired with the continued unrestricted submarine warfare by Germany, solidified the public sentiment that the United States needed to enter the war. Thus, the correct answer to the student's question is C: they were outraged that Germany planned to support Mexico if it tried to reclaim land from the United States.