Answer: a. the unpopularity of Russian involvement in World War I
Step-by-step explanation:
In the early 20th century, Russia as an empire was on the verge of unrest due to measures by the various emperors and nobles that had led to the poor being poorer, hungrier and angrier.
This led to various political movements such as the Communists and Socialists forming in an effort to develop the country and for the country to move from the abusive absolute monarchical system of Governance.
After Russia joined the first world war on the side of the Allies, the Germans defeated several of their armies in the Eastern front. Early victories Russia had achieved were the morale boosters that had kept Russia going and with that morale gone, the people rioted due to starvation, poor working conditions and widespread poverty and the army that was normally able to keep them down was on the verge of mutiny.
This led to a revolution in March 1917 that was then overthrown in October of the same year by the Communists who then took power and took Russia out of the war.