What is an ex post facto law?

An ex post facto law refers specifically to legislation that makes an act illegal or increases the penalties for an act that was committed before the law was passed. Therefore, a law that allows punishment for actions that were legal at the time they were performed fits this definition. The U.S. Constitution prohibits ex post facto laws to ensure fairness, preventing individuals from being penalized for actions that were lawful when taken. This principle is critical in protecting individuals' rights against retrospective laws that could unjustly punish or criminalize past behavior.

The other options do not accurately describe the concept of ex post facto laws, as they either suggest changes in penalties or implications for legal benefits that do not address the fundamental issue of applying laws retroactively to past actions.