In human blood group inheritance

(a) I^A and I^B are codominant

(b) I^A and I^B are dominant over i

(c) I^A is dominant over I^B

The alleles I^A and I^B produce a slightly different form of the sugar while allele i does not produce any sugar. 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.