Which command is used to extend a formation during preparatory drill?

The command "Fall In" is used to initiate the assembly of personnel into a formation. When soldiers are instructed to "Fall In," they are directed to take their positions and ensure they are properly aligned with one another. This command is critical in the preparatory drill as it sets the stage for extending a formation by gathering soldiers in an organized way.

In the context of a preparatory drill, extending a formation often involves ensuring that troops are evenly spaced and correctly positioned. "Fall In" provides the command necessary to start this process, allowing leaders to then instruct soldiers on how to adjust their positions further, if needed.

The other options, while they serve specific purposes in military drills and formations, do not specifically pertain to the initial act of gathering soldiers into an organized formation framework. Commands like "Advance," "March Forward," or "Align Left" pertain to movements or adjustments after the formation has already been established. Thus, understanding the fundamental role of "Fall In" is crucial in the context of preparatory drills.