ABO blood group is an example of :

The correct answer is Option (2) → Multiple alleles

Option (1) Single allele — Incorrect. One allele alone cannot account for the three forms I^A, I^B and i. The ABO system needs more than two alleles of one gene, which is multiple allelism.

Option (2) Multiple alleles — Correct. The gene I has three alleles (I^A, I^B, i); a person carries any two, giving six genotypes and four blood groups.

Option (3) Single gene — Incorrect. It is indeed one gene, but what the ABO system shows is that this gene has more than two alleles.

Option (4) Two genes — Incorrect. Only one gene, I, controls the ABO blood groups; its three alleles give the six genotypes. No second gene is involved.

NCERT Extract:
Co-dominance – “Till now we were discussing crosses where the F1 resembled either of the two parents (dominance) or was in-between (incomplete dominance). But, in the case of co-dominance the F1 generation resembles both parents. A good example is different types of red blood cells that determine ABO blood grouping in human beings. ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene. The gene (I) has three alleles I^A, I^B and i. The alleles I^A and I^B produce a slightly different form of the sugar while allele i does not produce any sugar.”