Final answer:
In total darkness and confined spaces, firefighters find the flashlight the most useful part of their personal protective equipment as it allows for visibility in otherwise non-seeable conditions.
Step-by-step explanation:
The part of structural fire fighting personal protective equipment (PPE) that is most useful in total darkness and confined spaces is the flashlight. A flashlight is essential for a firefighter to navigate through the darkness, locate victims, and find safe egress paths in an environment where visibility is near zero.
Helmets, gloves, and boots are also critical components of a firefighter's PPE, but it's the flashlight that provides the illumination needed in dark conditions. Firefighters also rely on other technological advancements such as fire-resistant clothing made from materials like Nomex, and breathing apparatus with design contributions from NASA technology, to ensure safety and efficiency during operations.