When Israel demanded a king, the LORD told Samuel the people had rejected whom?

Answer: The LORD Himself

God told Samuel, "It is not you they have rejected, but they have rejected Me as their king" (1 Samuel 8:7).

“And the LORD said to Samuel, “Listen to the voice of the people in all that they say to you. For it is not you they have rejected, but they have rejected Me as their king.”— 1 Samuel 8:7 (BSB)