A man with blood group B marries a woman with blood group A and their first child is having blood group B. What is the genotype of child?

The correct option is C $I^B$$I^O$

The blood group of the man is B. The genotype of the man will be I${}^B$ I${}^O$ or I${}^B$ I${}^B$. The woman has the blood group A so, the genotype will be I${}^A$I${}^O$ or I${}^A$I${}^A$. The first child has blood group B. The gamete I${}^B$ from the male will combine with I${}^O$ from the female to give the blood group B. So, the genotype will be I${}^B$I${}^O$.

Thus, the correct answer is option C.