Answer:
The correct answer is C. Vladimir Lenin and the communists overthrew the Russian tsar.
Step-by-step explanation:
The Russian Revolution took place in 1917, when Vladimir I. Lenin and the Red Army faced and overthrew the tsar and the White Army.
A is incorrect because the reforms were instaured by Gorbachev during the decades of 1980 and the beginning of the 1990's. B was a result of the reforms so it is also incorrect, while C is incorrect because Stalin ruled the USSR between 1922 and 1952, so his tenure started 5 years after the Revolution.