What action must be taken by the referee to signal a throw-in on the end line?

The correct action to signal a throw-in on the end line is to blow the whistle. This signal is imperative as it clearly indicates to all players, coaches, and spectators that a stoppage in play has occurred and that a specific play, the throw-in, is being initiated. The whistle serves as an auditory cue that helps to ensure that everyone is aware of the new phase of play, thereby promoting order and understanding on the court.

While hand gestures play an important role in communicating many aspects of the game, the blowing of the whistle is the primary method for conveying the message that play will resume with a throw-in. Additionally, calling out a player's name or raising both arms in the air may not provide the clarity needed for all participants on the court regarding the change in the game's flow. Thus, the whistle is essential for effective communication and management of the game.