Multiple allelism : Generally in an individual/population, only two alleles of a trait govern the character, but in case of ABO blood group, three alleles IA, IB and i are found to govern blood group in human population.
Co-dominance : Allele IA and IB when present in an individual, both being dominant express their own types of sugars/traits (no marks for the second step if two alleles are not given correctly).
Detailed Answer:
In human blood group there are four possible phenotypes A, B, AB & O. These blood groupings are controlled by gene I. There are three, instead of normally two, alleles of this gene namely IA, IB & i which control these four blood groups. Hence it is an example of multiple allelism.
Out of these three alleles IA and IB are dominant over i. Each person in a population possesses any two of the three alleles one each of the two parents. When allele IA and IB both are present together in an individual, both being dominant express equally and independently and hence are codominant.
Thus, human blood is a good example of both multiple allelism as well as codominance.