Which is a basic feature of a civilization? reduction of religious rituals reliance on tribal associations self-sufficient economy social class hierarchy

the answer is social class hierarchy 
What distinguishes a civilization from a simple group of people is the separation of works/job (specialization of labors)  that humans do so every needs of all members in that civilizations is fulfilled.
 During this separation, some people will hold more so-called 'important'  jobs compared to others (such as kings, business leaders, high-priests, etc) and will naturally formed a social hierarchy among them.