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 group 3 was given the task of observing the debate to ascertain the true attitude of the members of groups 1 and 2.  None of the 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 done 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.