A 90-year-old home care client's son has been designated to make decisions regarding the client's medical care when the client is no longer able to do so. As the client nears the end of life, the son is consulted on an ever-increasing basis. What legal instrument activates the son's decision-making designation?

Durable Power of Attorney.

Explanation:

Durable power of attorney:-It is the power that lets a trusted person to handle your affairs if you somehow become incapable of taking decisions by yourself. So durable power of attorney remains effective even when you become mentally incompetent.

Hence the answer to this question is Durable power of Attorney.