Correct answer: German troops became demoralized and eventually surrendered in the face of the arrival of fresh US troops.
The United States had been providing some support to the Allies prior to its own entry into the war. But when the US joined the effort fully with a declaration of war, that not only meant the arrival of fresh US troops but a commitment of even more American dollars to the Allies' war effort. The American commitment greatly boosted the morale of British and French troops on the Western front and demoralized the German troops.
As to the other answers ...
- Russia did pull out of the war, but that was because it went through a revolution and change of government in its own country, not because of American involvement. Russia's fight was with Germany on the Eastern front, whereas the US entered the war on the Western front.
- No, Mexico did not attack the United States -- although there had been a German diplomatic telegram that had suggested that as a possibility.
- Ottoman forces did their fighting in the Balkans and in the Middle East, and were not involved in the war on the Western front.
