The Poona Pact was a significant agreement that was signed between Mahatma Gandhi and B.R. Ambedkar. It took place in September 1932 and aimed to address the issue of separate electorates for the depressed classes (now known as the Scheduled Castes).

Option 1: Muhammad Ali Jinnah was a prominent leader of the All India Muslim League and had different goals and concerns compared to Ambedkar, so he is not the correct answer.

Option 2: Lord Irwin was the Viceroy of India at the time, and although he was involved in negotiations, the Poona Pact was actually signed between Gandhi and Ambedkar.

Option 3: Subhash Chandra Bose was a renowned freedom fighter, but he was not directly involved in the Poona Pact.

Therefore, the correct answer is Option 4: B.R. Ambedkar.