Which daughter of Saul was promised to David but given to Adriel instead?

Answer: Merab

Saul offered his older daughter Merab but gave her to Adriel of Meholah. His younger daughter Michal loved David and married him.

“Then Saul said to David, “Here is my older daughter Merab. I will give her to you in marriage. Only be valiant for me and fight the LORD’s battles.” But Saul was thinking, “I need not raise my hand against him; let the hand of the Philistines be against him.””— 1 Samuel 18:17 (BSB)