What legal document would allow a person to make decisions for another in case of incapacity?

The durable power of attorney is the correct choice because it is a legal document specifically designed to grant an individual the authority to make decisions on behalf of another person in the event that the latter becomes incapacitated. This document is crucial for ensuring that someone's wishes are followed regarding financial and health care decisions when they are unable to communicate or make those choices themselves. The person designated as the agent or attorney-in-fact can manage the affairs of the individual who has granted this authority.

In contrast, a living will addresses end-of-life medical care choices but does not appoint someone else to decide on behalf of the individual. A subpoena is a legal document that compels a person to appear in court or produce evidence; it is not related to decision-making capacity. Informed consent refers to the process of ensuring that a patient understands and agrees to medical treatment, which does not pertain to granting decision-making authority in cases of incapacity.