The correct answer is Lucknow Session, 1916 .

At the Lucknow Session of the Indian National Congress in 1916, Mahatma Gandhi was apprised of the problems faced by the peasants in Champaran. This session is known for its significance in Indian history as it marked the reunion of the moderate and the extremist factions within the Indian National Congress, who had been divided since the Surat Session of 1907.

At the Lucknow Session, Mahatma Gandhi was not yet a prominent leader within the Congress. However, he had established himself as a leader in the Indian community in South Africa, where he had fought for the rights of Indian immigrants.

During the session, the leaders of the Congress discussed various issues, including the problems faced by peasants in different parts of India. It was during this session that Gandhi was informed about the plight of the Champaran peasants by Rajendra Prasad, who would later become the first President of India.

Gandhi was deeply moved by the plight of the peasants and decided to travel to Champaran to investigate the situation himself. This led to the famous Champaran Satyagraha of 1917, where Gandhi and his followers used nonviolent resistance to protest against the British government and the indigo planters.

In conclusion, the correct answer to the question is option 4) Lucknow Session, 1916. This session marked the reunion of the moderate and extremist factions within the Indian National Congress and also led to Mahatma Gandhi’s involvement in the Champaran Satyagraha and his later work in the Indian independence movement.