a) the son will not be affected by rickets (0% chance)
b) the daughter will have rickets (100% chance)
Explanation:
Since only the father has rickets and it is a X-linked dominant trait, the mother has two normal X chromosomes. The X chromosome received from the mother by the son will not result in the rickets since the father will have contributed the Y chromosome and that does not have the gene for rickets. In case of the daughter, she will receive the defective X chromosome from her father, which will cause her to have the rickets disease.