Final answer:
The nurse demonstrates the principle of fidelity by fulfilling the commitment to provide the patient with information about the diagnostic test.
Step-by-step explanation:
When a nurse returns with an explanation after promising to find out more information about a diagnostic test, the nurse is acting under the principle of fidelity. This ethical principle obligates healthcare professionals to be faithful to the commitments they make to their patients and to honor the trust placed in them. The nurse's action reflects a dedication to this principle, ensuring that the patient's need for understanding and information is met.