The blood group of one parent is AB and the other parent is O. When combining the blood types of the parents, the possible blood group of their child could be A or B.

Option 1, "A or B", is the correct answer because the child can inherit either the A blood type from the AB parent or the B blood type from the O parent.

Option 2, "A or B or O", is not correct because the child cannot inherit blood type O from parents who do not have blood type O.

Option 3, "A or AB or O", is not correct because the child cannot inherit blood type AB from parents who do not have blood type AB.

Option 4, "A or B or AB or O", is not correct because the child cannot inherit blood type AB from parents who do not have blood type AB.