The gene controlling the ABO blood groups in humans is:

The correct answer is Option (3) → I

Option (3) I — Correct. The ABO blood groups are controlled by the gene I, which has three alleles: I^A, I^B and i. This is an example of multiple alleles.

NCERT Extract:
Co-dominance – “… 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.”