The prominent leader of the Congress Socialist Party (CSP) among the options provided is Acharya Narendra Dev, who was associated with the organization during the Indian independence movement. Let`s examine the reasoning behind this choice and provide additional information to support it.

1. M. N. Roy: While M. N. Roy was indeed a significant figure in the Indian independence movement, he was not directly associated with the Congress Socialist Party. Roy was one of the founders of the Communist Party of India and later went on to establish the Radical Democratic Party.

2. Ganesh Shankar Vidyarthi: Ganesh Shankar Vidyarthi was a renowned journalist and freedom fighter, but he was primarily associated with the Indian National Congress and not specifically with the Congress Socialist Party.

3. Pattam Thanu Pillai: Pattam Thanu Pillai was a prominent leader from the Indian National Congress and played a crucial role in the freedom struggle, particularly in the state of Kerala. However, there is no substantial evidence to suggest his direct involvement with the Congress Socialist Party.

4. Acharya Narendra Dev: Acharya Narendra Dev was a highly influential leader of the Indian independence movement and a prominent member of the Congress Socialist Party. He played a key role in advocating for socialistic ideals within the Congress party and was an ardent supporter of land reforms, workers` rights, and socialist principles.

Acharya Narendra Dev was actively involved in various progressive and socialist movements in India. He championed the cause of the peasantry and workers, fought against feudalism, and emphasized the importance of economic and social justice. Dev was known for his commitment to non-violence and his efforts to bridge the gap between the Congress party and socialist ideologies.

During his tenure as the President of the Congress Socialist Party, Acharya Narendra Dev played a significant role in shaping the organization`s policies and advocating for socialist principles within the broader Indian National Congress. He worked towards addressing the concerns of marginalized sections of society and ensuring their representation within the Congress party.

In conclusion, among the options provided, Acharya Narendra Dev was the prominent leader associated with the Congress Socialist Party. His contributions to the Indian independence movement, commitment to socialist ideals, and active involvement in progressive movements make him a notable figure in India`s political history.