Psalm 51 was written after which prophet came to David?

Answer: Nathan

Its title says David wrote it "when Nathan the prophet came to him after his adultery with Bathsheba."

“For the choirmaster. A Psalm of David. When Nathan the prophet came to him after his adultery with Bathsheba. Have mercy on me, O God, according to Your loving devotion; according to Your great compassion, blot out my transgressions.”— Psalm 51:1 (BSB)