Final answer:
In Huntington's 'Clash of Civilizations,' the highest cultural grouping and broadest level of social identity is referred to as a civilization.
Step-by-step explanation:
According to Samuel P. Huntington's Clash of Civilizations, a social identity that reflects the highest cultural grouping and the broadest level of identity is best described as a civilization. Huntington argues that civilizations are the grandest level of identity, surpassing other forms such as lineage orders and nations. Lineage orders are societies structured around extended family groups, with leadership typically provided by elders. Nations, on the other hand, are communities bound by a common language, origin story, and shared values and norms. But civilizations encompass larger cultural and historical commonalities that can include multiple nations.