What is the first command to transition a unit from a normal line formation to an extended rectangular formation?

The correct command to transition a unit from a normal line formation to an extended rectangular formation is "Extend to the left, march." This command specifically instructs the squad to expand the line to the left, allowing for a more stretched and rectangular formation by positioning individuals further apart.

The phrase "Extend to the left" indicates the direction in which the unit should spread out, and "march" signals the execution of this movement. This command is vital in military drills to ensure that the formation is organized and that soldiers maintain their alignment while transitioning to a wider formation.

In contrast, the other options do not adequately facilitate this specific transition. "Step forward, march" does not address the need to expand the formation horizontally, instead implying a forward movement without consideration of spacing. "Arms downward, move" is typically a command related to the position of arms but does not initiate any formation change. "Left, face" instructs soldiers to turn left but does not guide them in adjusting the formation for an extended configuration. Therefore, "Extend to the left, march" is the only command that successfully accomplishes the desired transition.