Explanation:

- Verrier Elwin (Option 3) advocated the policy of isolation for the tribals in India.

- He believed that tribal communities should be protected from the influence of mainstream Indian society and be allowed to develop at their own pace.

- G.S. Ghurye (Option 1), on the other hand, supported the policy of assimilation; he believed tribals should integrate with the mainstream culture.

- M.N. Srinivas (Option 4) is known for concepts like 'Sanskritization' and advocated integration, not isolation.

- Bailey (Option 2) is not primarily associated with the policy of isolation or mainstream tribal policy debates in India.

Correct Answer: Option 3 - Elwin