Father of a child is haemophilic and the mother is a carrier for the defective gene, the probability of the child having haemophilia is

The correct option is B $50\%$
Explanation –
$\begin{array}{ccc}\text{Male}\downarrow /\text{Female}\to & X^H& X\\ X^H& X^H{X}^H& X^{H}X\\ Y& X^{H}Y& XY\end{array}$
Out of 4 children 1 son and daughter would be haemophilic, 1 daughter can be a carrier and 1 son completely normal. So 2 out of 4 children are haemophilic. Hence the probability is 0.5.