The correct answer is A) Zimbardo did not use a control group.
The major flaw in the research design of the Stanford prison experiment was that Zimbardo did not use a control group.
A control group was necessary to study a particular variable in prison. He had no alternative group to compare and measure the results of the experiment.
Other mistakes he had were that he was part of the experiment. He participated as the prison superintendent. He was an authoritative figure inside the experiment of the prison and influenced the behavior of his students. He should have a neutral position.
You know you could just type it into google right?
