Answer : 4. "Dr . B.R. Ambedkar "
Explanation :

Dr. B.R. Ambedkar, the first Law Minister of independent India and a key framer of the Indian Constitution, was responsible for mass religious conversion. He converted to Buddhism along with a large number of his followers in 1956 as a form of protest against caste discrimination and to seek social and economic equality. This mass conversion event is known as the "Dhamma Chakra Pravartan Din" or the "Dhamma Chakra Anupravartan Diwas."