The correct answer is the fourth one: the casualty rate of their own army was high.
Russia had one of the largest numbers of casualties during World War I, it which were around 1.7 million combat deaths and missing in action. It was only inferior to the number of German casualties at the end of the war. But, unlike Germany, Russia was a poor and underdeveloped country. The economy of the country depended chiefly on agriculture and millions of peasant workers who also thickened the military ranks. The loss of men in the war and the complicated economic panorama that Russia was facing generated a negative feeling of the people toward the Russian participation in the war. It also pushed the workers to oppose the imperial government and the royal family, which lost legitimacy due to these external and internal problems. One year later, in October 1917, a massive and revolutionary outbreak of workers, led by Vladimir Lenin and the Communist party, overthrown the imperial government and established the first Communist government known as the Union of Soviet Socialist Republics. One of the first measures the new government took was to withdraw from the war.
