Final answer:
The probability of a grandson being color-blind from a colorblind grandfather and a grandmother with no color blindness gene, with subsequent marriages to only normal individuals, is nil. Therefore, the correct option is A.
Step-by-step explanation:
Since color blindness is an X-linked recessive condition, a man (XY) who is colorblind will pass on his Y chromosome to his sons and an X chromosome to his daughters. A woman with normal vision who carries no color blindness gene (XX) will always give one of her normal X chromosomes to her children. If the daughter (XX) from the marriage mentioned in the question marries a normal man (XY), their children would either have a normal X from their mother and a normal X or Y from their father (0% chance of color blindness in their sons, and daughters would not be colorblind but are not carriers either).
As such, the F1 progeny of a colorblind man and a woman with normal vision will not carry the colorblind gene if we assume the woman has no color blindness alleles. If the F1 female progeny does not carry the gene for colorblindness, when she marries a normal male, none of their children can inherit the trait. Therefore, the probability of their grandson being color-blind is nil (a).