Final answer:
The Polynesian practice of royal endogamy to preserve mana within the ruling class is a manifest function, which is the explicit reason people give for their customs.
Step-by-step explanation:
The belief that high levels of mana mark people as sacred in some Polynesian communities, and the associated practice of royal endogamy to preserve this force within the ruling class, is best described as a manifest function. A manifest function is the overt or intended explanation people provide for their customs. In this case, the explanation is that royal endogamy is practiced to maintain the sacredness and power within the ruling lineage.