In which situation would a pitcher be deemed to have committed a balk?

A pitcher is deemed to have committed a balk when he fails to deliver a pitch after stopping his motion. This situation is critical because it disrupts the fairness of the game, as runners may attempt to advance while the pitcher is in a paused state. The definition of a balk is intended to prevent pitchers from gaining an unfair advantage over base runners, and stopping their motion without delivering the ball is a clear violation of this rule.

In contrast, other options present scenarios that generally do not constitute a balk. For example, stepping back off the pitcher's plate before throwing does not necessarily lead to a balk; it may simply be a part of the pitcher's routine. Being within the plane of the pitcher's plate is standard positioning and does not lead to a balk unless specific other actions accompany it. Lastly, having the pivot foot completely in front of the pitcher's plate might be a legal position depending on how the pitcher delivers the pitch; it would only be a balk if improper actions follow. Thus, the situation described in the chosen answer accurately aligns with the definition of a balk.