Final answer:
The nurse is putting into action the ethical theory of utilitarianism by prioritizing the severity of injuries to ensure that the most critical patients receive care first.
Step-by-step explanation:
The nurse is putting into action the ethical theory of utilitarianism. Utilitarianism states that the rightness of an action is determined by its consequences, specifically the greatest happiness for the greatest number of people. In this scenario, the nurse is prioritizing the severity of injuries to ensure that the most critical patients receive care first, which aligns with the utilitarian principle of producing the greatest overall happiness.