Final answer:
Personnel accountability systems assist in keeping track of firefighters at an emergency scene.
Step-by-step explanation:
The BEST description of personnel accountability systems is option D, which states that these systems assist in keeping track of firefighters at an emergency scene. Personnel accountability systems are used in emergency response situations, particularly in firefighting, to ensure the safety and accountability of all personnel involved. These systems help track firefighters' locations, monitor their activities, and ensure that everyone is accounted for during an emergency.