Which of the following refers to explosive obstacles that are not properly detonated?

The term that refers to explosive obstacles that are not properly detonated is "UXO," which stands for Unexploded Ordnance. UXO refers specifically to military munitions that have been primed, fused, or armed, and that remain unexploded after the conclusion of a military engagement or conflict. These munitions can pose significant dangers, as they may still be capable of detonating if disturbed or handled improperly.

In contrast, IEDs (Improvised Explosive Devices) are typically devices that are intentionally set to detonate, often in an unpredictable manner, and do not fall under the category of unexploded ordnance. Mines, while they can also be devices that remain undetonated, are usually designed to explode upon triggering, making them more specific in function than UXO. Bombs generally refer to munitions intended to be deployed in a manner that detonates them upon landing or impact, rather than remaining as unexploded obstacles.

Hence, UXO is the most precise term for explosive hazards that have not detonated as intended and thus represent a risk for individuals in the area.