The correct answer is C. The war that preceded the communist revolution in Russia was World War I.
The term Russian Revolution brings together all the events that led to the overthrow of the Tsarist regime and the prepared establishment of another, communist, between February and October 1917, which led to the creation of the Russian Soviet Federative Socialist Republic. The Tsar was forced to abdicate and the old regime was replaced by a provisional government during the first revolution of February 1917 (March in the Gregorian calendar, since the Julian calendar was in use in Russia at that time). In the second revolution, in October, the Provisional Government was eliminated and replaced with a Bolshevik (communist) government, the Sovnarkom.
The Russian Revolution was focused, originally, around Petrograd (now St. Petersburg). In the chaos, the members of the imperial parliament assumed control of the country, forming the Russian provisional government. The army leadership felt they did not have the means to suppress the revolution and Nicholas II, the last Russian emperor, abdicated. The soviets (workers' councils), which were led by more radical socialist factions, initially allowed the provisional government to rule, but insisted on a prerogative to influence the government and control various militias. The Russian revolution took place in the context of the hard military reverses suffered during the First World War (1914-1918), which left much of the Russian army in a state of mutiny.
