Answer: D) a mental set
Explanation:
His mental set is giving him a procedure of wrong ways into the solving-problems. He is having mental set where he is convinced that the children will listen to him and that they will do their homework only if he is giving them threats which means that he is trying to control them with anger and fear.
Since his mental set is in a wrong way, he does not believe that kindness will bring him more productivity in class.