Vladimir Lenin was the leader of the Bolsheviks.

Background
Before we dive into the answer, let's provide some background information. The Bolsheviks were a faction within the Russian Social Democratic Labor Party (RSDLP) that was formed in 1912. This party was split into two main factions - the Bolsheviks and the Mensheviks. The key difference between the two factions was their approach to revolution.

Lenin's Role in the Bolsheviks
1. Leadership: Vladimir Lenin emerged as the leader of the Bolshevik faction. He played a crucial role in shaping the ideology and strategies of the Bolsheviks.
2. Revolutionary Vision: Lenin believed that the revolution should be led by a vanguard party comprising professional revolutionaries who would guide the working class towards a socialist revolution. This vision was in contrast to the Menshevik belief that the revolution should be a broad-based movement involving multiple social groups.
3. Role in the October Revolution: Lenin's leadership was instrumental in the success of the October Revolution in 1917. He provided the ideological framework and direction for the Bolsheviks, which led to the overthrow of the Provisional Government and the establishment of Soviet power.
4. Implementation of Socialist Policies: After the revolution, Lenin played a pivotal role in implementing socialist policies in Russia. He led the Bolshevik government and initiated various reforms, including land redistribution, nationalization of industries, and the establishment of the Soviet state.

Other Options
a) Mensheviks: The Mensheviks were a rival faction within the Russian Social Democratic Labor Party. They advocated for a more moderate and gradual approach to revolution, in contrast to the Bolsheviks' revolutionary stance. Lenin and the Bolsheviks often clashed with the Mensheviks over ideological and strategic differences.

b) Nihilists: Nihilism was a philosophical and cultural movement that emerged in Russia during the 19th century. Nihilists rejected traditional values and institutions and often advocated for radical change. While Lenin and the Bolsheviks shared some revolutionary goals with the nihilists, they were distinct political entities with different ideologies and methods.

d) Red Shirts: The Red Shirts were a paramilitary group in Italy led by Giuseppe Garibaldi during the 19th century. They played a significant role in the Italian unification movement. However, they are not directly related to Lenin or the Bolsheviks in Russia.

In conclusion, Vladimir Lenin was the leader of the Bolsheviks, a faction within the Russian Social Democratic Labor Party. He played a crucial role in leading the Bolsheviks to power, implementing socialist policies, and shaping the course of the Russian Revolution.