b. 25%(Ref: Nelson's 20/e p 642; Ghai 8/e p 641)Color blindness is an X-linked recessive disorder.In X-linked disorders, if father is normal and mother is a carrier, 50% of children will inherit defective X chromosome.Out of these, the daughters (having 1 normal and 1 defective X chromosome) will act as carriersWhile, all sons, inheriting the defective X chromosome will be affected by colour blindness. Hence, 25% of children will be affected.