Answer 
The correct answer would be B) 50 %.
In humans, the blood group is determined by three alleles  and
 and  .
.
 are co-dominant whereas
 are co-dominant whereas   is recessive to other two.
 is recessive to other two.
Thus, the genotype of a person with blood group O would be  .
.
The genotype of a person with blood group B would be either  (homozygous condition) or
 (homozygous condition) or  (heterozygous condition).
 (heterozygous condition).
The crosses are shown in the image below.
There is only one condition in which the person can have offspring with blood group O that is, when the other parent is  .
.
In this condition, the probability of an offspring to have blood group O is 50%. 
In other condition, the probability of an offspring to have blood O is 0%.