ABO blood grouping in human beings cites the example of:

A- Incomplete dominance

B- Co-dominance

C- Multiple allelism

ABO blood group system in humans is controlled by gene (I). This gene has three alleles- $I^A$, $I^B$ and i. Therefore, ABO blood grouping provides a good example of multiple alleles.

$I^A$ and  $I^B$ are completely dominant over i, in other words when $I^A$ and i are present only $I^A$expresses (because i does not produce any sugar), and when $I^B$and i are present $I^B$ expresses. But  when $I^A$ and  $I^B$are present together they both express their own types of sugars: this is because of co-dominance.