a man having blood group A marries a woman with blood group O and their daughter has blood group O. is this information enough to tell whether A or O is dominant? why?
No it is not enough.
This information is insufficient to determine the dominant blood group. During cross breeding, even recessive traits can express themselves phenotypically. For example, if a heterozygous male AO mates with OO female, then there is a probability of the child inheriting OO genotype. So, you cannot determine the dominant blood group in this case.
The ABO blood type is controlled by a single gene with three alleles: i, IA, and IB. The IA allele gives type A, IB gives type B, and i gives type O. As both IA and IB are dominant over i, only ii people have type O blood. Individuals with IAIA or IAi have type A blood, and individuals with IBIB or IBi have type B. A type A and a type B couple can also have a type O child if they are both heterozygous (IBi,IAi).
So in case of a heterozygous male with IAi genotype marrying a ii genotype female, the child has a probability of inheriting ii phenotype, which results in O blood group. The A blood group is actually dominant over O, but here we cannot determine the dominancy based on the given example.