An ex post facto law is a type of law that establishes that a certain act that was legal before, is no longer in the present, can also apply a type of conviction or infraction to said actions or somehow change evidence during a conviction for make it effective.
This is exactly what happens in the example, where everyone who has used fireworks for a period of time where that was still legal is now being condemned.
This type of law is prohibited by the constitution.
Given this information we can say that the correct answer is: an ex post facto law.
