The united states responded to the sinking of the lusitania and other
passenger ships by
(a) immediately declaring war on germany
(b) forcing the creation of the sussex pledge
(c) entering a formal alliance with the allied powers
(d) issuing the zimmerman telegram
(e) declaring war on mexico

C- entering a formal alliance with the Allied powers 

b. Wilson called the action an act of barbarism but continued his push to keep the United States out of war.
In 1915, the passenger ship Lusitania was sunk by German submarines, killing 128 Americans. Wilson demanded the end to the unrestricted use of submarine warfare. Eventually, in March 1916, after the sinking of the passenger ship Sussex, Germany issued the Sussex Pledge, stating three important promises: (1) German submarines would no longer target passenger ships; (2) merchant ships would not be attacked unless they were known to be carrying weapons; and (3) provisions would be made for personnel and passengers of merchant ships to get off the ship safely prior to its destruction.