Which task helps prevent friendly forces from entering contaminated areas?

Marking is crucial in CBRN reconnaissance operations as it involves identifying and delineating the boundaries of contaminated areas. By clearly marking these areas, units are given visual cues and warning signals about the hazards present, allowing them to steer clear of potentially dangerous zones. This task is essential for maintaining the safety of friendly forces, as it provides immediate and recognizable indicators that signify contamination, thus preventing accidental entry into these hazardous environments.

In contrast, detection focuses on identifying the presence of hazardous materials, while monitoring involves ongoing observation of the area to assess changes in contamination over time. Warning, on the other hand, is about informing units of existing dangers but does not inherently involve demarcating these dangers in an observable manner. While all of these tasks contribute to overall safety and situational awareness, marking specifically addresses the direct need to delineate and avoid contaminated areas.