Which is the best example of an ex post facto law?

An ex post facto law is one that retroactively changes the legal consequences of actions that were committed before the law was enacted. This principle is outlined in the U.S. Constitution, which prohibits such laws to ensure fairness and justice in the legal system.

The scenario where Fred Fern receives a speeding ticket on the day before a new speed limit goes into effect embodies this concept. The enforcement of the new speed limit does not apply to actions taken before it was legally established. Therefore, penalizing Fred for speeding based on the forthcoming law violates the ex post facto prohibition, as it applies retroactively to conduct that occurred before the law's enactment.

Other options do not exemplify ex post facto laws. For instance, the new tax imposed on future transactions applies only going forward and does not affect past actions. The introduction of stricter gun control laws also impacts future behavior and does not impose penalties retroactively. Lastly, a law allowing retroactive health care coverage acts to provide benefits for past actions rather than imposing penalties, which is different from the punitive nature of ex post facto laws.