The correct answer is option 3, Mahatma Gandhi. The Second Round Table Conference was held in London in 1931, and Mahatma Gandhi was the representative of the Indian National Congress during this conference.

Let`s examine the other options to understand why they are incorrect:

Option 1: Madan Mohan Malviya was an important figure in the Indian independence movement, but he did not represent the Indian National Congress at the Second Round Table Conference.

Option 2: C. Rajagopalachari, also known as Rajaji, was a prominent leader of the Indian National Congress, but he did not represent the party in the Second Round Table Conference.

Option 4: Vallabhbhai Patel, also known as Sardar Patel, was another significant leader of the Indian National Congress, but he was not the representative of the party at the Second Round Table Conference.

Therefore, the correct answer is Mahatma Gandhi.