Explanation:

- Harijan Sevak Sangh: Founded in 1932, this organization aimed to eradicate untouchability and promote social welfare among Harijans (Dalits) in India.

- Option 1: Jawaharlal Nehru

- First Prime Minister of India.

- Key figure in the Indian independence movement.

- Not the founder of Harijan Sevak Sangh.

- Option 2: Mahatma Gandhi

- Correct Answer: Founder of Harijan Sevak Sangh.

- Championed the cause of Dalits and worked for their upliftment.

- Focused on social reforms alongside the independence movement.

- Option 3: Jyotiba Phule

- Social reformer known for his efforts towards the education of women and lower castes.

- Did not establish Harijan Sevak Sangh.

- Option 4: BR Ambedkar

- Principal architect of the Indian Constitution.

- Advocated for Dalit rights but was not the founder of Harijan Sevak Sangh.