Explanation:

- The unit of voluntary membership in social organisation, according to M.N. Srinivas, refers to a group in which people join by choice, not by birth.

- Option 1: Caste – This is an ascribed group; membership is determined by birth, not voluntary.

- Option 2: Lineage – This too is decided by descent or birth, not choice.

- Option 3: Family – Also involuntary; one is born into a family.

- Option 4: Faction – Factions are groups formed around a leader or interest, where membership is based on choice and loyalty rather than birth.

- According to M.N. Srinivas, the faction is the true unit of voluntary membership in Indian village society. He observed people could freely join or leave factions based on their interests.

Option 4: Faction is the correct answer.