ABO blood grouping in human beings is an example for

The correct option is D

All of these

ABO blood grouping in human beings is controlled by three alleles namely $I^A,I^{B}and{I}^O$. These are basically the three alternative forms (alleles) of the gene I. Hence it is an example for multiple allelism. Among the three alleles $I^A$ and $I^B$ are dominant over $I^O$/i, which is why this case is also an example for dominance. Further, $I^A$ and $I^B$ are both expressed and therefore are said to be equally dominant (codominant). Hence it is an example of codominance too.