A Recessive Allele on the X Chromosome Is Responsible for Red-Green

A recessive allele on the X chromosome is responsible for red-green color blindness in humans. A woman with normal vision whose father is color blind marries a color-blind male. What is the probability that this couple's first son will be color blind?

A) 1/4
B) 1/2
C) 2/3
D) 3/4