Which of the following could potentially generate unexploded ordinance (UXO)?

The potential for generating unexploded ordnance (UXO) primarily arises from military operations, making combatants the most relevant choice. Combatants are involved in direct engagement and hostilities that utilize munitions, which, if not properly detonated during an operation, can leave behind unexploded devices. These remnants pose significant hazards in both the immediate area and for future activities, including civilian clearance operations.

In contrast, civilians normally do not engage in activities that involve the use of explosives in a military context, while supply units and maintenance crews are typically involved in supporting roles that do not directly result in the detonation of munitions. Supply units focus on the logistics of moving and storing munitions, and maintenance crews are responsible for upkeep of equipment, which, though critical, does not inherently create the risk of UXO in the manner that combat actions do.