To determine which group best represents a basic unit, we need to clarify what the "basic unit" refers to. If the basic unit in question is a "community" or "society," then "people who live together under a sovereign government" fits that definition best, as it encompasses the structure and organization of a society.
If we are considering different types of communities or social groups, any of the other options could also qualify depending on the context. However, based on a general societal definition, the most fitting response would be:
people who live together under a sovereign government.