The correct answer is (a).
The Harijan Sevak Sangh was founded in 1932 by Mahatma Gandhi to uplift the condition of the untouchables in India. It was originally called the All India Anti-Untouchability League, but was renamed the Harijan Sevak Sangh in 1933.
The All India Depressed Classes Association was founded in 1924 by Dr. B.R. Ambedkar to fight for the rights of the untouchables. The Depressed Classes Association for Social Reforms was founded in 1927 by M.C. Rajah to promote social reform among the untouchables. The Association of Untouchables was founded in 1930 by E.V. Ramasamy Naicker to fight for the rights of the untouchables.
All of these organizations played an important role in the upliftment of the untouchables in India. However, the Harijan Sevak Sangh was the most successful organization, and it is still active today.