Which condition is one of the four main conditions requiring boresight?

Boresighting is a crucial process in ensuring that a weapon system's aiming devices are aligned correctly with the weapon itself. One of the four main conditions that typically necessitates boresight is a hard landing. This situation can significantly impact the aircraft or the weapon system's structural integrity and alignment. A hard landing may result in misalignment between the weapon's sighting system and its barrel, which could affect accuracy in future engagements.

In the context of military operations, ensuring that the weapon system is correctly boresighted after such an incident is essential for maintaining operational effectiveness and ensuring accurate targeting. The other conditions listed, while they may have an impact on equipment operation or effectiveness, do not specifically relate to the immediate need for boresighting in the same critical way as a hard landing does.