B: observers said they were unable to identify the debaters' true
    attitudes because the debaters had been randomly assigned to argue a certain position.

    Incorrect.
In section, group 1 was assigned to argue that the prison study was ethical, group 2 was assigned to argue that the prison study was unethical, and the class was given the task of observing the debate to ascertain the true attitude of the members of groups 1 and 2.  Very few members of the observer group said that were unable to identify the debates' true attitudes because the debaters had been randomly assigned to argue a certain position, as their instructions were to try the best they could to figure out the true attitudes members of group 1 and 2 held.  Perhaps they should have done or said what is contained in answer B, but they did not.  Social psychology often focuses on a descriptive model of behavior, which means it describes what people actually DO, as opposed to a normative model, which describes what people should do, or what is best to do in a certain situation.  In question 11, we see that we are looking at behavior from a descriptive perspective (what people did- commit the fundamental attribution error), instead of the normative perspective (what people should do- say that they were unable to identify the debates' true attitudes because the debaters had been randomly assigned to argue a certain position.