Membership in caste involves strict rules about marriage, where marriage is restricted to member of the group. This is known as:

Endogamy

The correct answer is option 2: Endogamy

Endogamy refers to the practice of marrying within a specific social group, caste, or community. In the context of caste, individuals are expected to marry within their own caste, and often within specific sub-castes as well.

Other options:

• Polygamy: The practice of having more than one spouse at the same time.

• Exogamy: Marrying outside one's own group or caste.

• Monogamy: The practice of having only one spouse at a time.