a.The allele is X-linked
Women have two X chromosomes while men have one X and one Y.
In the case of a recessive mutation on the X chromosome passes from the mother to the baby boy, the boy will express the recessive allele. Even though it's recessive, and in allosomes both alleles need to be present, when it comes to this situation the boy only has one X chromosome so it's indifferent if the gene is recessive or dominant.