What is the probability of having children with 'O' blood group,...

The genotype of the mother, who is heterozygous for 'A' blood group, is $I^{A}i$.

The genotype of the father, who is heterozygous for 'B' blood group, is $I^{B}i$.

A cross between the mother and father ($I^{A}i \times I^{B}i$) produces the following possible genotypes in the offspring:

$I^{A}I^{B}$ (AB blood group)
$I^{A}i$ (A blood group)
$I^{B}i$ (B blood group)
$ii$ (O blood group)

Out of the four possible combinations, only one results in the 'O' blood group ($ii$).

The probability of having a child with 'O' blood group is $\dfrac{1}{4}$.

In percentage, this is $\dfrac{1}{4} \times 100\% = 25\%$.

Answer: $25\%$