The stanford prison study illustrates that people: a. will obey orders even when they violate their principles. b. behave in accord with their roles in a situation. c. placed in all-male groups will create dominance hierarchies. d. are innately violent unless constrained by society.

I believe the answer is: b.behave in accord with their roles in a situation

In the study, two group of people that are not familiar with one another are assigned with either prisoners or the guard role.

As it turned out, despite the realization that they're just doing a mock situation,the subjects that assigned as guards actually do or say something derogatory to the subjects that assigned as prisoners.