Answer: The correct answer is-
ABO blood group shows multiple allelism that is has more than two alleles. AB alleles show co-dominance and O allele is recessive allele.
As per the information in the question, Mrs. Smith has blood type A, her father has blood type A ( genotype could be AA/AO), and her mother has blood type B ( genotype would be BO only). This means that she has inherited A allele from her father and O allele from her mother to make the blood type A. Thus, genotype of Mrs. Smith would be AO.
Mr. Smith has blood type AB ( that is genotype would be AB).
So, the genotypes of these parents would be-
Mrs. Smith- AO
Mr. Smith- AB
When these parents are crossed, they will have 25% probability of having child with blood type AB.