Which of the following is an example of multiple allelism?

$A$ couple has four children with blood groups $A, AB, A,$ and $O$ respectively. What are the genotypes of the parents' blood groups?

$A$ child with a mother of $O$ blood group and a father of $A$ blood group will have which of the following blood groups?

Mother is homozygous for blood group $B$,and father has blood group $A$ (genotype unknown). Therefore,the possible blood groups in the progeny are:

If the blood group of the mother is $A$ and the blood group of the father is $O$,what may be the blood group of the progeny?

$ABO$ blood grouping in humans is controlled by the gene $I$. It has three alleles: $I^A, I^B,$ and $i$. Due to these three different alleles,six different genotypes are possible. How many phenotypes can be formed from these?