What is the closest range bracket after which an observer must send refinement corrections for mortars if the last round is within a specified distance?

The correct answer pertains specifically to the standard practices for adjusting mortar fire. After the last round of fire, observers must send refinement corrections if the target distance falls within a specified range bracket. This range is important because it ensures that the observer quickly adjusts to maintain accuracy, which is crucial for effective fire support.

In this context, 100 meters is the closest range bracket that necessitates sending refinement corrections. This means that if the last round was fired within 100 meters of the target, it is vital for the observer to provide adjustments to account for any inaccuracies and to ensure that subsequent rounds land on target. This practice helps avoid collateral damage and ensures that the mortar fire is effectively neutralizing the intended threat.

The chosen answer reflects the standard operating procedure for determining when an observer needs to make refinements to ensure effective fire support is provided to ground troops in close proximity to the target area.