First and foremost, supervisors are B. TEACHERS.
To become an effective supervisor, one must know all the processes involved in the group he or she is supervising. If the supervisor notices that there is an error in the performance of the tasks assigned, he or she must teach the proper way of completing said task.
The supervisor must first become the teacher of the people under his supervision. This is to ensure that everyone under his supervision is working accordingly and effectively. If the supervisor does not know how his unit should perform, then, the unit will produce inferior output which in turn will become detrimental to company's performance in the long run.