Choice of membership in particular cliques and crowds is often determined by the degree of within members of the group.

The correct answer is the degree of similarity between group members. 

Members of a group who share similar preferences, beliefs and values are more likely to have a unanimous opinion on whom, should and should not belong to their group or clique. For instance, in a high school, a clique or group of cheerleaders who have lunch together everyday are unlikely to let in a non-cheerleader who plays in jazz band into their group.