What document outlines a patient's treatment preferences when they are terminally ill?

The correct answer is the Living Will. A Living Will is a specific type of advance directive that explicitly details a patient's wishes regarding medical treatment in the event that they become unable to communicate their preferences due to a terminal illness or irreversible condition. This document expresses the types of medical interventions the patient desires or does not desire, such as resuscitation or the use of life-sustaining measures, ensuring that their treatment aligns with their personal values and wishes.

While other options provide important roles in medical decision-making, they serve different purposes. A Power of Attorney grants someone the authority to make financial or legal decisions but does not specifically outline medical treatment preferences. An Advance Directive is a broader term that encompasses various documents including both Living Wills and Health Care Proxies, but it does not focus solely on treatment preferences. A Health Care Proxy designates a person to make medical decisions on behalf of the patient if they are unable to do so themselves, but it does not itself outline specific treatment preferences.

Therefore, a Living Will is the most direct means through which patients can communicate their treatment choices when facing terminal illness.