Answer: C) people define themselves by their group affiliations.
Explanation: Social identity can be simply defined as the perception of an individual on who they are based on the membership they belong to.
Social identity theory is a theory about (in group) who we are and (out group) who they are, the in group will always look for issues to use against the out groups, there by tarnishing the image of the out groups and enhancing theirs.
So option (C); people define themselves by their group affiliations best illustrates social identity theory.