In the context of estate planning, the option that serves as a complement to a will is a Durable power of attorney.
However, it is important to note that while all of the listed options (living will, trust, durable power of attorney, and health-care proxy) are related to legal and healthcare decisions, the durable power of attorney specifically allows someone to make legal and financial decisions on your behalf if you become unable to do so.
A living will outlines your wishes regarding medical treatment, a trust is used to manage and distribute assets, and a healthcare proxy is designated to make healthcare decisions if you are unable to do so.
Let me know if you need further clarification or additional resources!