Final answer:
The fiduciary duty at play is the duty of confidentiality.
Step-by-step explanation:
The fiduciary duty at play in this situation is the duty of confidentiality. Gary's client's concern about privacy and her desire to not have her listing posted on the MLS demonstrate her expectation of confidentiality. Despite Gary's explanation of the potential impact on marketing the property, he must respect and uphold his duty to keep her information confidential.