The UN said that two-thirds of children in Ukraine \_\_\_\_ because of the conflict, mostly within the country.

主詞 two-thirds of children 為複數，動詞需用複數形式 have；且兒童是受衝突影響而被迫流離失所，需使用被動語態。結合過去發生且對現在造成影響的語境，使用現在完成被動語態 have been displaced 完全正確。