A man whose father was colour blind marries a woman who had colour blind mother and normal father. What percentage of male children of this couple will be colour blind.

The correct option is B $50\%$

As color blindness is an autosomal recessive genetic disorder, for it is present at X -chromosome. Thus, according to the situation given in the question, a man whose father was color blind (will be, i.e, XY normal) marries a woman whose mother was color blind and father was normal (i.e, this woman will be a carrier) according to the cross given in the first figure.

Thus, when marriage will happen between a normal man and a carrier woman, in that case, the percentage of a male child to be colorblind is 25% (this can be easily observed from the second figure).

So, the correct answer is '50%'.