Human ABO blood type is determined by a single gene that comes in 3 distinct alleles: IA , IB , and i. The IA and IB alleles are each dominant to the i allele, but are codominant with each other, meaning that both phenotypes are expressed in the heterozygote. The Punnett square below shows a cross between two parents with different blood types. Drag the genotypes and phenotypes from the left to correctly complete the Punnett square.

given, mother and father are heterozygotes.

let, father has B blood group and mother has A blood group.

then their heterozygous  genotypes are --

father = iB, iO  and mother = iA, iO

punnett square will be,

        iB        iO

iA     iA iB     iA iO

iO     iB iO    iO iO

the genotypes of their progeny will be, iAiB ,  iAiO ,iBiO and iOiO

and phenotypes will be 3 are heterozygous and 1 is homozygous.

b

is correct hope this   also i learnd this in middle school