A man whose father was colour blind and mother is normal marries a woman who had a colour blind mother and normal father. What percentage of male children of this couple will be colour blind?

The correct option is B 50%
The gene of colour blindness is recessive X linked trait (represented as X${}^e$) and shows criss-cross inheritance. Since, the man has colour blind father and normal mother, the genotype of the man is XY. Again, the woman has colour blind mother and normal father and so the genotype is X${}^e$X. The cross between the couple is given below:

Out of two male children, one is colour blind. Hence, 50% male children will be colour blind.