Final answer:
The main focus of the investigation was d. to see the extent to which participants would engage in behavior they knew to be socially unacceptable.
Step-by-step explanation:
The experiment aimed to test the power of assigned roles to control participants' behavior and examine the influence of the situation on behavior. The study demonstrated how social roles and the power of the situation can significantly impact human behavior. The main focus of the investigation in the Stanford Prison Experiment was to see the extent to which participants would engage in behavior they knew to be socially unacceptable.