In the $ABO$ system, of blood group which of the following is example of co-dominance ?

Three children in a family have blood types $O$, $AB$ and $B$ respectively. What are the genotypes of their parents?

$ABO$ blood groups in humans are controlled by the gene $I$. It has three alleles ­$ I^A, I^B$ and $i$. Since there are three different alleles, six different genotypes are possible. How many phenotypes can occur?

Mother $B$ homozygous, father $A$ unknown, therefore, possible blood group in progeny is

$A$ : Blood group phenotype is controlled by presence or absence of antigens present on surface coating of $RBC$.
$R$ : These antigens are of three types and found in the oligosaccharides rich head regions of a glycophorin