Concept:The span of control is a management principle about how many subordinates one supervisor can manage effectively.Explanation:In an organisation, every superior has a limited capacity to supervise and guide staff.The term that refers to the number of staff a superior officer can efficiently supervise is the span of control.A wider span means more subordinates report directly to one manager, while a narrower span means fewer.Delegation of authority, division of labour, and unity of command do not measure the count of staff under one superior.Answer:D. span of control.