When a male child with genetic disorder is born to a physically normal couple, this could be due to a

The correct option is D X - linked recessive trait
Given that an asymptomatic couple have a male child with a genetic disorder. This disorder cannot be Y-linked, as the father in unaffected. It cannot be autosomal dominant as neither of the parents are affected. Dominant traits will always express in the bearer. This also rules out the possibility of X-linked dominance. So, the only possible scenario is that of X-linked recessive trait as this can be inherited from an unaffected carrier mother by the male offspring.