Hey there! I'm happy to help!
1917 was during World War 1, and since the U.S. was very powerful they did not want them being involved in it. So, they wanted Mexico to launch a separate attack on the U.S. as they say they want them to conquer some U.S. territories to distract the U.S. so they cannot interfere in the war. The USA was very powerful and Germany did not want them to join.
Therefore, the answer is c. attack the United States.
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