What is required for a pitcher to make a legal pitch from the set position?

For a pitcher to make a legal pitch from the set position, the requirement that the pitcher must have his foot fully on the pitcher's plate is essential. This ensures that the pitcher is in an appropriate stance to initiate the pitch. When the foot is fully on the plate, it establishes a stable and legal position from which to throw the pitch effectively. This rule helps maintain fair play and consistency in the way pitches are delivered during the game.

In the context of other options, although factors such as the pivot foot not being in motion and both feet being on the ground may contribute to aspects of pitching technique, they do not specifically define the legality of the position as critically as having the foot fully on the pitcher's plate. The timing issue related to throwing the pitch within a specific timeframe pertains to other rules regarding pace of play but does not define the legal starting position for delivering a pitch. Thus, the primary and clear requirement for a legal pitch from the set position is having the pitcher’s foot fully in contact with the pitcher’s plate.