Alegal document that states a client's health-related wishes — such as a preference for pain management if the client becomes terminally ill — and also allows the client's adult child to direct the client's care, is:

advance directive.

Explanation:

Advance directive is a legal document that the client provides to their respective health care providers about their health care decision making and health related wishes.

It also provides the clients decision and wishes on how the clients wants to be treated at some particular conditions when diagnosed with certain illness. It provides the legal method s on how the clients needs to be treated at times.

Thus the answer is advance directive.

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