Mahatma Gandhi participated in the Second Round Table Conference held in 1931. The Round Table Conferences were a series of meetings held in London between 1930 and 1932 to discuss constitutional reforms in India. The conferences aimed to bring together representatives from various political groups in India, including the Indian National Congress and the Muslim League, along with British government officials, to find a solution for India`s future political structure.

Gandhi played a critical role in advocating for India`s independence and representing the interests of the Indian National Congress during the meetings. Although he attended the First Round Table Conference in 1930, he actively participated and presented his demands for India`s independence during the Second Round Table Conference. Therefore, option 2, "Second Round Table Conference, 1931," is the correct answer.