Final answer:
The nurse manager exhibits the ethical principle of beneficence by promoting a staff nurse to assistant manager of the medical unit.
Step-by-step explanation:
The ethical principle exhibited by the nurse manager in this situation is beneficence. The principle of beneficence states that we should act in ways that benefit others or that are for the good of others. By promoting the staff nurse to assistant manager, the nurse manager is recognizing the nurse's interest in taking on more responsibilities and providing them with an opportunity for growth and development. This benefits both the staff nurse and the medical unit as a whole.