What type of laws are prohibited by the Ex Post Facto clause?

The Ex Post Facto clause specifically prohibits laws that impose penalties for actions that were legal at the time they were committed. This means that if a law is enacted that criminalizes a behavior that was previously allowed, individuals cannot be prosecuted for that behavior after the fact. This principle serves to uphold fairness and justice in the legal system, ensuring that individuals have the protection of knowing that their actions are governed by the laws in place at the time those actions were taken, rather than being subject to later changes in the law that could retroactively penalize them.

On the other hand, the other options relate to different legal principles. The financial obligations of states involve contractual agreements and taxation, but they do not directly concern the Ex Post Facto clause. Restrictions on freedom of speech pertain to First Amendment rights, and establishing new taxes retroactively involves different legislative processes that do not fall under the Ex Post Facto prohibition. Thus, the heart of the Ex Post Facto clause is about safeguarding against unjust retroactive laws that penalize past actions.