What action should a referee take if a player who was fouled retaliates by slapping the opponent?

When a player retaliates by slapping an opponent, the appropriate action for the referee is to show a red card. This is because the act of retaliation, particularly one involving physical contact such as slapping, is considered violent conduct. The Laws of the Game recognize that any form of violent behavior, regardless of the provocation, warrants expulsion from the match to maintain player safety and uphold the integrity of the game.

By issuing a red card, the referee sends a clear message that retaliatory actions will not be tolerated, helping to deter such behavior in the future. It also emphasizes the importance of maintaining sportsmanship within the match. This action is consistent with the referee's responsibility to control the game and protect all players involved.

A yellow card would not be sufficient in this scenario as it is generally reserved for cautionable offenses or unsporting behavior rather than violent conduct. A warning would not address the severity of the incident nor ensure accountability for the player’s actions. Similarly, taking no action overlooks the potential escalation of violence and fails to uphold the standards of the game.