Final answer:
The leadership principle that ensures Marines feel part of the team by promoting efficiency and morale is 'Keep your Marines informed'. This encourages a collaborative and transparent environment where Marines are better equipped to take initiative and participate actively in achieving the unit's goals.
Step-by-step explanation:
The leadership principle that promotes efficiency and morale by making Marines feel that they are part of the team is c) Keep your Marines informed. This principle encourages transparency and communication, ensuring that team members understand the reasons behind certain tasks and the overall mission. By keeping everyone informed, leaders can foster a sense of inclusion and team spirit, which are crucial for maintaining high morale and efficiency within the unit. It aligns with the notion that informed Marines are more likely to remain willing to go above and beyond and participate actively than those who are not privy to important information. Moreover, it nurtures an environment where Marines can be resourceful and minimize the need for supervision, as they are well-acquainted with the mission's objectives and can thus make more informed decisions.