Final answer:
The nurse returning the call to a patient as promised is an example of the ethical principle of fidelity, which pertains to keeping promises and maintaining trust. The correct option is 2.
Step-by-step explanation:
When a nurse in a primary care office returns a telephone call by the end of the day to a patient who is concerned about the outcome of a laboratory test, the nurse is demonstrating the ethical principle of fidelity.
Fidelity involves keeping one's promises and being truthful, which in this context translates to honoring the promise to return the call and thereby maintaining trust in the healthcare provider-patient relationship.