When one of the parents has 'A' blood group and the other parent has 'O' blood group, then their child can have _____ blood group.

Either 'A' or 'O'

The correct answer is Option (4) → Either 'A' or 'O'

The blood group of an individual is determined by the combination of alleles they inherit from their parents.

If one parent has blood group A, their genotype can either be I^AI^A or I^Ai. If the other parent has blood group O, their genotype is ii.

Possible Combinations:

If the parent with blood group A has the genotype I^AI^A  the children will have genotype I^Ai (blood group A).

If the parent with blood group A has the genotype I^Ai the children could have either I^Ai (blood group A) or ii (blood group O).

Therefore, the child can have either 'A' or 'O' blood group, depending on the genotypes of the parents.