A girl of normal vision whose father was colourblind marries a man of normal vision whose father was also colourblind. Their sons would be (of total number of sons)

The correct option is A 50% colourblind

Colour blindness is an X-chromosome linked disorder.

Girl of normal vision whose father was colourblind, is a carrier of the disease with genotypes: XC Xc (normal: affected genes)
Man of normal vision whose father was colourblind has the genotypes XC Y

The possible genotypes of sons: XC Y (Normal vision), Xc Y (Colourblind), which means that there is a 50% probability of the son to be colourblind.