Colour blindness is a X-linked recessive disorder. Son gets its X-chromosome from mother; since the male is affected, he has got the Xc allele from his mother. According to question, his parents are normal which means that father does not carry the disease; presence of disease in father would have expressed due to its hemizygous genotype for X-chromosome. And mother is carrier who passed her affected allele to male. Father is normal, this rules out the disease to be inherited from paternal grandparents; options C and D are wrong. Father transmits its X chromosome to the daughters so the carrier mother have inherited her affected X chromosome from father which means that maternal grandfather’s genotype is XcY; option A is correct. If maternal grandmother was carrier; the mother of male would be diseased as she will get one affected allele from father and one from mother; but mother is carrier here, option B is incorrect. Correct answer is option A.