The best option from the list would be that the Treaty of Brest-Litvosk "ended Russia's involvement in World War I", since the Russians were going through a Civil War.
The treaty of Brest-Litovsk was signed on March of 1918, after months of negotiations, between the Central Powers (Germany, the Ottoman Empire, Austria-Hungary, and Bulgaria) and Russia in the context of World War I, and which effectively ended Russia´s participation in the war and stopped further invasions. Russia, at the time, was fighting its own civil war and this treaty brought certain stability to the Bolsheviks leading their civil war.
Sigmund Freud (writing between the 1890s and the 1930s) developed a collection of theories which have formed the basis of the psychodynamic approach to psychology.