Answer:
either A or O
Step-by-step explanation:
A blood type of a child is inherited from his parents. A child gets one of two ABO genes from each biological parent.
Mr. X has been accused by Miss Z of fathering her child. The child's blood type is AB and Miss Z's blood type is A.
If Mr. X has a blood group either A or O, then he can't be the father of the child.