When does an appeal become valid concerning a missed base?

An appeal concerning a missed base becomes valid during active play because it allows the defensive team to seek a ruling on a runner’s failure to touch a base properly, which is critical in the context of gameplay. Appeals must be made while the game is ongoing, as they are mechanisms for the defense to challenge the offensive team's actions directly related to base running.

This timeframe ensures that the situation is fresh and contemporaneous, allowing umpires to make decisions based on the current state of play. The other contexts mentioned, such as after all players have touched bases, when the ball is dead, or when the offensive team acknowledges the error, do not align with the active play requirement necessary for a valid appeal. Each of these situations implies either the conclusion of a play or a lack of immediate challenge, which undermines the purpose of the appeal in the flow of the game.