The rule that was part of the caste system is option C. A person of higher social standing could not marry a person of lower social standing. Caste is a form of social stratification characterised by endogamy, hereditary transmission of a style of life which often includes an occupation, ritual status in a hierarchy, and customary social interaction and exclusion based on cultural notions of purity and pollution.
Endogamy is known as in-marriage, custom enjoining one to marry within one's own group. The penalties for transgressing endogamous restrictions have varied greatly among cultures and have ranged from death to mild disapproval. When marriage to an outside group is mandated, it is referred to as exogamy.
Therefore, the correct answer is as given above
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