Father’s blood group is A and mother’s blood group is B. One of their…

The correct answer is Option (2) → 25%

Note: Option (3) 50% is also correct. The question does not give the parents’ genotypes: the answer is 25% when both parents are heterozygous (I^Ai × I^Bi) and 50% when one parent is homozygous (I^AI^A × I^Bi or I^Ai × I^BI^B).

The ABO blood group is controlled by the gene I, which has three alleles: I^A, I^B and i. I^A and I^B are co-dominant, so a person with both shows blood group AB. The allele i is recessive. So blood group A can be I^AI^A or I^Ai, and blood group B can be I^BI^B or I^Bi.

Both parents heterozygous: I^Ai × I^Bi. Gametes: I^A, i from the father and I^B, i from the mother.

One box in four is AB: 1/4 = 25%.

Option (1) NIL — Incorrect. An AB child has already been born, so the father carries I^A and the mother carries I^B. The chance of AB can never be zero.

Option (2) 25% — Correct. This is the chance when both parents are heterozygous, as the square shows.

Option (3) 50% — Also correct. If one parent is homozygous (say I^AI^A × I^Bi), the father gives I^A every time and the mother gives I^B half the time, so half the children are AB.

Option (4) 75% — Incorrect. No cross of an A parent with a B parent gives three AB children in four. The chance is 25%, 50% or, with both parents homozygous, 100%.

NCERT Extract:
Co-dominance – “… The gene (I) has three alleles IA, IB and i. The alleles IA and IB produce a slightly different form of the sugar while allele i does not produce any sugar. Because humans are diploid organisms, each person possesses any two of the three I gene alleles. IA and IB are completely dominant over i, in other words when IA and i are present only IA expresses (because i does not produce any sugar), and when IB and i are present IB expresses. But when IA and IB are present together they both express their own types of sugars: this is because of co-dominance.”