What should the referee do if the head coach of Team B receives a second caution?

When the head coach of a team receives a second caution, the appropriate action for the referee is to show a yellow card followed by a red card. In the context of the Laws of the Game, a player or team official who accumulates two cautions for unsporting behavior is considered to be committing a more serious offense, which warrants a dismissal from the game.

The first yellow card serves as a warning, while the second yellow card indicates that the coach's actions are not acceptable, leading to a red card, which results in ejection from the match. This emphasizes the importance of maintaining a level of respect and sportsmanship within the game. The red card signifies that the individual must leave the vicinity of the field and cannot participate any further in that match.

Other options such as only showing a yellow card or simply asking the coach to leave without a card do not follow the stipulated rules and procedures of officiating in soccer, which require a clear and formal method of sanctioning infractions. Sending a message to the coach instead also undermines the official nature of disciplinary actions in soccer. Thus, showing a yellow card followed by a red card is the correct and necessary course of action.