Aman with type ab blood is married to a woman with type o blood. they have two natural children, and one adopted child. the children's blood types are: a, b, and o. which child was adopted'

The child with the blood type O is most likely adopted.

First of all, blood type A and B are co-dominance and O is recessive. From this, we can conclude that the genotype of the man is AB, because only A and B blood together can make someone have blood type AB, and on the other hand, the woman should have the genotype of OO, as O is recessive, only all Os can result in the phenotype of blood type O. 

We can draw a Punnett square to find what are the possible blood types of the offsprings. From the image of the square, we can see all 4 possibilities are either type A or type B blood, as again, A and B are dominant over O. Therefore, it is impossible for an offspring from these 2 people to have a blood type O. So, the child who has blood type O is mostly adopted.