Apart from co-dominance, the ABO blood grouping is also a good exampl…

The correct answer is Option (2) → Multiple alleles

The ABO blood groups are controlled by the gene I, which has three alleles, I^A, I^B and i. I^A and I^B are co-dominant, as both express when present together. Because a single gene has more than two alleles governing the same character, ABO is also an example of multiple alleles. An individual carries only two of them, so the three alleles are seen only when a population is studied.

Option (1) Incomplete dominance — Incorrect. Incomplete dominance means the F₁ is in between the two parents, as in the pink snapdragon. No ABO blood group is an in-between blend of A and B.

Option (2) Multiple alleles — Correct. Three alleles of one gene (I^A, I^B, i) govern the same character, which makes the ABO system a good example of multiple alleles.

Option (3) Lethal gene — Incorrect. A lethal gene is one that causes the death of the organism carrying it. None of the I alleles is lethal; every combination gives a normal blood group.

NCERT Extract:
Co-dominance – “… Do you realise that the example of ABO blood grouping also provides a good example of multiple alleles? Here you can see that there are more than two, i.e., three alleles, governing the same character. Since in an individual only two alleles can be present, multiple alleles can be found only when population studies are made.”