On whose threshing floor did David build an altar to stop the plague?

Answer: Araunah the Jebusite

The prophet Gad told David to build an altar there. David bought the threshing floor and the oxen for fifty shekels of silver.

“And that day Gad came to David and said to him, “Go up and build an altar to the LORD on the threshing floor of Araunah the Jebusite.””— 2 Samuel 24:18 (BSB)