A man heterozygous for blood group B marries a woman having blood group O, then the percentage of progeny having blood group A would be75%50%25%Nil

The percentage of progeny having blood group A would be Nil.

Here's why:

A man heterozygous for blood group B means he has one allele for blood group B (I^B) and one allele for blood group O (i). His genotype is I^Bi.

A woman with blood group O has two alleles for blood group O (i). Her genotype is ii.

When they have a child, the child gets one allele from each parent. The possible combinations are I^Bi (which results in blood group B) and ii (which results in blood group O).

There is no possibility for the child to have blood group A because neither parent has an allele for blood group A (I^A). Therefore, the percentage of progeny having blood group A is 0%, or Nil.