Final answer:
Proverbs 31:1-9 is the sole instruction to a king in the Book of Proverbs, addressed to King Lemuel.
Step-by-step explanation:
The passage in the Book of Proverbs that is considered an instruction to a king is found in Proverbs 31:1-9. This instruction is addressed to King Lemuel, and it contains advice given to him by his mother. It is a set of moral and ethical guidelines aimed at guiding the king in his leadership and roles of governance. The passage emphasizes the importance of righteousness, justice, and advocating for those who are unable to defend themselves. Although specifics about King Lemuel's identity or reign are not well-documented outside of the biblical context, this proverbial advice is regarded as a universal benchmark for kings and leaders in exercising fair and compassionate rulership.