Explanation:

- Option 1: Irawati Karve

- She was an Indian anthropologist known for her study of kinship and social structure in India.

- Her work is influential but does not specifically address the clan as an exogamous unit.

- Option 2: Pauline Kolenda

- An American anthropologist known for her work on Indian culture and kinship systems.

- She did explore kinship but didn't define clans explicitly as exogamous divisions.

- Option 3: Fox

- Robin Fox, a known anthropologist, focused on kinship and social structures.

- Though he's contributed significantly to these fields, his works aren't specific to defining clans as exogamous divisions.

- Option 4: Rivers

- W.H.R. Rivers was a significant figure in anthropology who studied kinship.

- His works included defining clans as exogamous divisions of tribes.

- Correct Answer: Option 4, Rivers