What are ex post facto laws?

Ex post facto laws are those that make an action illegal retroactively, meaning that an individual can be punished for an act that was not considered a crime at the time it was committed. Therefore, the correct answer states that they are actions that cannot be made illegal after they have occurred. This principle is foundational in guaranteeing fairness in the legal system and is specifically prohibited by the U.S. Constitution.

The prohibition against ex post facto laws ensures that citizens have the right to know what is legal and illegal before they act. It protects individuals from being punished under laws that did not exist when the actions were taken, which helps to maintain a sense of legal certainty and justice. Other options do not capture this critical aspect of how ex post facto laws function within the legal framework.

For instance, laws that apply to future actions or increase penalties for future crimes do not pertain to the ex post facto standard, as they deal with prospective regulation rather than retroactive application. Similarly, regulations made after a crime is committed do not represent the essence of ex post facto laws because they imply changes without consideration of when the actions took place, thus not accurately reflecting the nature of retroactive laws.