The notion of purity, integral to the caste system is maintained by the practise of:

Endogamy

The correct answer is Option (3) - Endogamy

Endogamy refers to the practice of marrying within one's own social group, caste, or community. In the context of the caste system, endogamy is essential for maintaining purity within caste boundaries and ensuring that individuals marry within their own caste to preserve social hierarchy and avoid contaminating purity.