The correct answer is C) U.S. forces ended the stalemate and led to an Allied victory."
The chief result of U.S. participation in World War I was "U.S. forces ended the stalemate and led to an Allied victory."
US President Woodrow Wilson had decided to maintain the foreign policy of neutrality in World War 1, buy the sinking of the Lusitani ship and the interception of the Zimmerman telegram in which Germany asked Mexi for help in WW1, made President Wilson ask Congress for a declaration of war against Germany. The US entered World War 1 on April 6, 1917.