Explanation:

- Dual organisation, as a system, refers to the division of a society into two social groups, moieties, or halves that interact through marriage, rituals, and social duties.

- This form of functional stratification is most commonly seen among tribes, not castes.

- Castes are hierarchical systems mainly found in South Asia, and they involve endogamy and occupational specialization, but not division into dual moieties.

- Class refers to economic and social division, which is different from functional dual organisation.

- Ethclass refers to a subgrouping that combines ethnicity and class, but again, not dual social structuring.

- Tribes often organize themselves into two groups for balanced interaction, alliance, and marriage exchanges.

Correct Answer:

- Option:4, tribes