In which scenario does F1 legally turn his shoulders to observe a runner on first base?

In the context of observing a runner on first base, the correct scenario is that the pitcher can legally turn his shoulders to look at the runner whether the motion is abrupt or casual. This is important because it establishes that the act of turning to observe a runner is permitted as part of the pitcher's actions on the mound, regardless of the style of the turn.

Allowing for both abrupt and casual movements recognizes the pitcher's discretion in how they manage their focus and strategy on the field. It enables them to effectively monitor base runners while also maintaining their position and readiness for the pitch.

In contrast, the other scenarios imply restrictions on the pitcher's ability to observe base runners, which could impede their strategic options during the game. The scenarios involving stepping back from the pitcher's plate, being in the windup position, or delivering a pitch suggest specific actions that could distract from the primary focus of monitoring base activity, thus contributing to a less effective defense.