Which city did God say He had given to Lot's descendants, so Israel must not harass Moab?

Answer: Ar

"Do not harass the Moabites or provoke them to war... because I have given Ar to the descendants of Lot as their possession."

“Then the LORD said to me, “Do not harass the Moabites or provoke them to war, for I will not give you any of their land, because I have given Ar to the descendants of Lot as their possession.””— Deuteronomy 2:9 (BSB)