Q. 18.  Mother and father of a person with 'O' blood group have 'A' and 'B' blood group respectively. What would be the genotype of both mother and father?

a.  Mother is homozygous for 'A' blood group and father is heterozygous for 'B'

b.  Mother is heterozygous for 'A' blood group and father is homozygous for 'B'

c.  Both mother and father are heterozygous for 'A' and 'B' blood group respectively

d.  Both mother and father are homozygous for 'A' and 'B' blood group respectively

(c).  When the cross is carried out between heterozygous father (for blood group B) and heterozygous mother (of blood group A) to get four children with different blood groups.

All the four blood groups are controlled by three allelic genes IA ,IB, i and thus it shows the phenomenon of multiple allelism. Both IA  and IB is dominant over i. However, when together, both are dominant and show the phenomenon of codominance forming the blood group AB. Six genotypes are possible with a combination of these three alleles.
Thus, other options are wrong or incorrect.