A woman with normal vision, but whose father was colour blind, marries a colour blind man. Suppose that the fourth child of this couple was a boy. This boy

The correct option is D May be colour blind or may be normal vision
The woman will be a genetic carrier - XX${}^c$ and man will be X${}^c$Y.
The possible offspring are : XX${}^c$, XY (normal son), X${}^c$X${}^c$ and X${}^c$Y (color blind son).
50% chance for the son to be colour blind.