The term "illegal" is specifically used to denote an act that is against the law. When something is classified as illegal, it means that it is prohibited by legislation and can lead to penalties, including fines or imprisonment, depending on the severity of the offense. This term is clear and unequivocal in its meaning, making it the appropriate choice to describe actions or behaviors that violate established legal standards.
In contrast, "permissible" refers to actions that are allowed or permitted within certain guidelines or regulations, so it does not convey any notion of illegality. "Authorized" means something that has been officially approved, which again does not imply anything against the law. "Legitimate" implies legality and acceptance within legal or accepted standards, further distancing it from the concept of being unlawful. Therefore, the term "illegal" is the only choice that directly correlates to acts that defy legal statutes.