The correct option is "an ex-post facto law."
The ex post facto means after the fact or incident. The law punishes someone for an action that does not used to be a crime when the action was performed. For example, you play in front of Congress and next day legislators make a law which criminalizes the same. So next day you will be arrested under the ex post facto law. Under the US constitution this law has been declared unconstitutional as fairness has been the fundamental principal of US constitution and democracy.