Final answer:
The nurse's action of providing medication promptly after promising to do so represents the moral principle of fidelity, which relates to keeping promises and being truthful.
Step-by-step explanation:
The scenario you have described best represents the moral principle of fidelity. When the nurse promptly returned with the medication within five minutes after promising to do so, she fulfilled the commitment made to the client. The nurse's action aligns with the principle of fidelity, which involves keeping promises and being truthful. It is part of the five prima facie duties identified by Ross, which also include reparation, gratitude, promoting the overall good, and non-maleficence.