Final answer:
The nurse should know the applicable State laws when determining the legality of an advance directive.
Step-by-step explanation:
When determining the legality of an advance directive, the applicable laws that the nurse should know are State laws. Each state has its own set of laws regarding advance directives, including living wills, Do Not Resuscitate (DNR) orders, and health care proxies. These laws outline the requirements for creating and executing advance directives, as well as the legal validity and enforceability of these directives. Therefore, it is important for nurses to be familiar with the specific state laws when assessing the legality of an advance directive.