What is the minimum diameter required for a safety zone relative to flame height?

The correct choice is based on established fire safety guidelines which dictate that a safety zone must be determined relative to the height of the flames produced by a fire. The rationale is that a safety zone needs to be large enough to account for thermal radiation and potential fire spread.

A safety zone that is four times the flame height provides a buffer that mitigates the risks associated with heat and embers that can emanate from the fire. This distance helps ensure that individuals, equipment, and structures are adequately protected from radiant heat and other hazards associated with wildfires or large structure fires.

The multiples of flame height derive from empirical observations and testing that show that heat intensity diminishes with distance, and thus a larger safety zone is necessary for higher flames to ensure adequate survival and safety of responders and bystanders. Therefore, utilizing the four times factor helps establish a more effective and safer operational environment during firefighting activities.