Final answer:
Russia signed a peace treaty with Germany as a result of the Russian Revolution, allowing Germany to concentrate on the Western Front until the U.S. joined and the Allies triumphed (b).
Step-by-step explanation:
As a result of the revolution in Russia, Russia signed a peace treaty with Germany. This event took place after the Russian Revolution of 1917 which brought down the Tsarist autocracy and led to the rise of the Bolsheviks under Vladimir Lenin.
Weakened by the internal strife and the demands of World War I, Lenin's government sought to extricate Russia from the conflict, leading to the signing of the Treaty of Brest-Litovsk in March 1918. This peace treaty allowed Germany to focus its military efforts on the Western Front, although it did not ensure a German victory, as the U.S. joined the war and the Allied forces eventually pushed Germany back, leading to the Armistice of November 1918.