Answer: a. the treaty of brest-litovsk.
The treaty of Brest-Litovsk was signed between the Russian government (now made up of Bolsheviks) and the Central Powers (German Empire, Austria-Hungary, Bulgaria, and the Ottoman Empire). It effectively ended Russia's participation in the war, as they agreed to stop further invasion. The change of government was the motivation, as Soviet Russia defaulted on all of Imperial Russia's commitments to the Allies. It also ceded hegemony over the Baltic Seas to Germany, ceded the province of Kars Oblast to the Ottoman Empire and recognized the independence of Ukraine. However, the treaty was annulled by the Armistice of November 11, 1918.
