which man is their father? [ ]is their father? their＝ [ ]

       答案：【Who】is their father? their father=【the children's father】
理由如下：
“Which man is their father?”中文：哪个男人是他们的父亲?
同义句为：谁是他们的父亲? “谁”即是who,Who is their father?
their father意思是“他们的父亲”,他们=[the  children]（孩子们）,他们的父亲=[the children's father]