Which of the following best defines a living will?

A living will is specifically designed to outline an individual's preferences regarding medical treatment in the event that they become unable to communicate those wishes themselves. This legal document provides guidance to healthcare providers and loved ones about the types of medical interventions or life-sustaining measures the individual desires or does not desire, typically in situations where they are facing terminal illness or severe incapacitation.

By having a living will, individuals can ensure that their medical treatment aligns with their personal values and preferences, particularly in critical situations such as end-of-life care. This is essential for respecting a person's autonomy and decisions concerning their own health and wellbeing.

The other options, while related to health care, address different aspects: a medical history record contains details of a patient’s past health conditions and treatments, guidelines for nurses focus on procedures and best practices in patient care, and prescriptions specifically pertain to the authorization of medication for patients. Each of these serves important purposes in healthcare but does not capture the essence of what a living will represents.