What instrument grants authority to act as an agent or attorney in fact for another?

The instrument that grants authority to act as an agent or attorney in fact for another is the power of attorney. This legal document allows one individual to appoint another to make decisions on their behalf, which can include managing financial matters, making legal decisions, or handling various personal affairs. The significance of the power of attorney lies in its ability to provide the agent or attorney in fact with the authority to perform these acts as if they were the principal themselves.

In contrast, a durable power is a specific type of power of attorney designed to remain in effect even if the principal becomes incapacitated. This does not change the fundamental definition of what a power of attorney is, but it adds a layer of protection in certain circumstances. A health care proxy is a specific type of power of attorney that pertains solely to making medical decisions on behalf of another person when they are unable to do so. Lastly, a consent form is generally utilized to give permission for specific actions or treatments but does not grant the extensive authority that a power of attorney provides. Each of these alternatives serves a different purpose, highlighting why the power of attorney is the correct choice for granting broad authority to act on someone else's behalf.