What signal indicates an offside offense?

The correct signal that indicates an offside offense is the raising of the flag by the assistant referee. This action serves as a visual cue to both the players and the referee that an offside infringement has occurred. When the assistant referee identifies a player in an offside position at the time the ball is played to them, they raise their flag to alert the referee and stop the play.

This flag signal is crucial for maintaining the flow of the game and ensuring that all players are adhering to the offside rule, which is intended to promote fair play and discourage players from gaining an unfair advantage by positioning themselves too close to the opponent's goal.

In contrast, the other options do not specifically communicate an offside offense. While a whistle from the referee can signify various types of infractions or interruptions in play, it does not uniquely indicate an offside situation. A verbal call from the referee may also address various aspects of the game but is not the standard method of signaling for offside. Showing a red card indicates a serious foul or misconduct, which is entirely separate from the offside rule.