What is an ex post facto law?

An ex post facto law is defined as a law that penalizes individuals for actions that were not considered illegal at the time those actions were taken. This principle is rooted in the legal system's commitment to fairness and justice, ensuring that individuals are not punished retroactively for behaviors that were legal when they occurred. Such laws violate the principle of legality, which posits that laws must be clear and published before they can be enforced, allowing citizens to understand what is permissible.

The concept of ex post facto laws is specifically prohibited in the U.S. Constitution, reflecting a fundamental legal protection against arbitrary government action. This safeguard helps maintain a stable legal environment, whereby individuals can confidently engage in activities knowing that they align with laws existent at that moment in time.

The other options pertain to different legal concepts: changing the constitution is a process of amendment, laws applying only to future acts represent the principle of prospective application, and restrictions on free speech relate to First Amendment rights, none of which define an ex post facto law. Thus, option B captures the essence and legal implications of an ex post facto law accurately.