Which of the following is a common indicator of fire behavior that triggers an immediate response?

A flare-up is a sudden increase in fire intensity and spread, indicating a significant change in fire behavior that can pose immediate danger to firefighters and the surrounding environment. This phenomenon typically occurs when a fire encounters new fuels or weather conditions that enhance its intensity, such as increased wind or a drop in humidity. The unpredictable nature of a flare-up necessitates immediate action to protect personnel and mitigate the risk of loss.

In contrast, a controlled burn refers to a deliberate fire set to manage vegetation and reduce fuel loads; it is not an indicator of imminent danger but rather a planned and monitored operation. Pre-attack involves preparations made prior to engaging with a fire, which does not require an immediate response. Active rehab focuses on the recovery and well-being of fire personnel, not on addressing fire behavior itself. Thus, flare-ups are a clear and urgent indicator that necessitates quick intervention to ensure safety on the fireground.