The United States declared war on Germany a as a result of German U-boats sinking of four unarmed American merchant vessels b as a result of treaty obligations c in response to demands by American munitions makers d after Mexico signed an alliance with Germany

Yes. That is correct. German U boats were sinking American merchant ships (that was called unrestricted submarine warfare) and the Zimmerman Telegram you were trying to explain. The Zimmerman telegram explained that Mexico attacks America on Germany's side (The Central Powers). If Germany wins, Mexico will get its territory lost to the US (Arizona, New Mexico and Utah.).