Final answer:
The cause of the Bolshevik establishment of the Soviet Union was Vladimir Lenin's promise to solve Russia's economic problems through communism option C.
Step-by-step explanation:
The cause of the Bolshevik establishment of the Union of Soviet Socialist Republics was revolutionary leader Vladimir Lenin's promise to solve Russia's economic problems through communism. After the Bolshevik seizure of power in the 1917 and a civil war broke out between the Red Army of the Bolsheviks and the White Army representing the various groups opposing them and Eventually, the Bolsheviks emerged the victorious, and Russia was reorganized as the Soviet Union in the early 1920s.