During play, when does a ball become 'live'?

A ball becomes 'live' when the umpire calls 'play ball.' This signal signifies the start of the action in the game, allowing players to engage and take their positions actively. Until that call is made, the game remains in a non-active state, and no plays can be executed.

The moment the umpire signifies that the game is in play, all actions, including pitching, hitting, and running, can commence. This ruling is crucial for maintaining the flow and organization of the game, as it provides a clear and consistent signal for everyone involved.

The other options do not represent the correct moment when a ball becomes live. For example, while the pitcher being on the mound is related to the game, it does not mean the ball is live until the umpire's call is made. Similarly, a ball being hit by the bat can only occur after it is live, and having a runner on base does not determine the ball's status either. Thus, the correct understanding is centered around the umpire's call as the definitive moment signifying the game's active status.