Explanation:

- Option 1: Dipankar Gupta

- An Indian sociologist known for his studies on Indian society and social stratification.

- However, he is not commonly associated with theories on purity, pollution, or untouchability.

- Option 2: Srinivas

- M.N. Srinivas was an influential Indian sociologist.

- While he contributed to the study of caste and social structure, he did not primarily focus on purity and pollution in relation to untouchability.

- Option 3: Y. Singh

- Yogendra Singh is a noted Indian sociologist.

- He is recognized for his work on modernization and social change, but not specifically for theories on purity and pollution in caste.

- Option 4: L. Dumont

- Louis Dumont was a French anthropologist known for his work on caste in India.

- He argued that notions of purity and pollution are central to the Hindu caste system and are fundamentally tied to the institution of untouchability.

- Your choice is correct. L. Dumont indeed considered purity and pollution as integral to untouchability.