The term 'X-rated challenge' has a number of synonyms that can be used to describe it. One such term is adult content challenge, which refers to any content that is meant for an adult audience. Another synonym is explicit challenge, which implies that the content is graphic and leaves little to the imagination. A third option is mature challenge, which typically indicates that the content is intended for an older audience and may be too intense for younger viewers. Whatever term you choose to use, it's important to understand that these challenges can be controversial and should be approached with care.