The primary function of a living will is to:

The primary function of a living will is to declare a person's healthcare preferences. A living will is a legal document that outlines an individual's wishes regarding medical treatment in situations where they may not be able to communicate their decisions themselves, such as in cases of terminal illness or incapacitation. It allows individuals to specify the types of life-sustaining treatments they do or do not want, thus ensuring their healthcare decisions are respected even if they cannot voice them at that time.

This contrasts with other options: summarizing healthcare coverage typically pertains to insurance documents, assigning a medical power of attorney involves designating someone else to make healthcare decisions on your behalf, and evaluating physician performance relates to assessing a doctor's qualifications or treatment effectiveness. None of these functions directly address the personal preferences in medical care that a living will encompasses.