ABO blood groups are controlled by the --

The correct answer is Option (3) → I gene

Option (3) I gene — Correct. ABO blood groups are controlled by the gene I, which has three alleles, I^A, I^B and i. A, B and O are the names of blood groups, not separate genes.

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.”