The correct answer is option 1: Haemophilia. Haemophilia is a human genetic disorder that is sex-linked, meaning it is carried on the sex chromosomes. Specifically, it is linked to the X chromosome. Since males have one X and one Y chromosome, they are more likely to be affected by sex-linked disorders carried on the X chromosome because they only have one copy of the X chromosome. Females, on the other hand, have two X chromosomes, which means they have a backup copy of the X chromosome that can compensate for any mutations or abnormalities. Cystic fibrosis, albinism, and PKU are not sex-linked disorders, as they are caused by mutations in autosomal genes, which are found on non-sex chromosomes.