_______ refers to marriage within a unit which could be a kin group, caste or a group living in the same locality.

Endogamy

The correct answer is Option (4) → Endogamy

Types of marriages in acient India:

Endogamy refers to marriage within a unit - this could be a kin group, caste, or a group living in the same locality.
Exogamy refers to marriage outside the unit.
Polygyny is the practice of a man having several wives.
Polyandry is the practice of a woman having several husbands.