I would say the best answer would be option D "Peter’s parents have not done enough explaining about the true nature of TV monsters so that he can better understand." The reason for this treatment failure is that Peter's parents are not explaining to him how TV works and how the monsters are fake which is why he still thinks monsters are real. So, in order for this treatment to work it's best to have the parents make their child understand how TV works and how monsters are made up.
Hope this helps.