Answer:
The city of Kirkuk would be the most difficult one to resolve.
Step-by-step explanation:
Kirkuk is ethnically diverse where the main four groups are Arabs, Iraqi Turkmen, Kurds, and Assyrians. It would be difficult to resolve as all four ethnic groups lay claim to the city as their own for a variety of historical and cultural reasons. The Kurds claim it as a part of Kurdistan. The Turkmen claim it to be a Turkmen city. The Arabs claim it as their own having arrived in Kirkuk in the 7th century. The Assyrians lived here even before the Arabs did.