How is a batter’s foot determined as being inside the batter's box?

A batter's foot is determined to be inside the batter's box if no part of either foot touches outside the box. This definition aligns with the essential rules of maintaining a proper stance within the batter's box. The emphasis here is on ensuring that the entire foot remains within the designated area during the act of batting.

If even a small part of a foot extends beyond the front edge or a side of the box, it could be interpreted as the batter being outside the box, which can lead to penalties or make the batter subject to certain strike rules. This understanding is crucial for batters as it affects their eligibility to swing at pitches and their overall legality while in the box.

The other choices do not align with the primary rule. For example, suggesting that both feet must be secured inside the box or that it's permissible for one foot to simply be on the lines does not capture the complete requirement that any contact with the outside area invalidates the situation. The mention of an airborne foot might also lead to confusion, as the active stance and contact with the box are more relevant than just where the foot lands after a jump or shift in position.