What characteristic does a foam solution require for effective fire suppression?

A foam solution needs the ability to form a blanket over the fuel for effective fire suppression. This characteristic is crucial because the foam blanket acts as a barrier, isolating the fuel from the oxygen in the atmosphere. By covering the fuel, the foam smothers the fire, preventing re-ignition and suppressing the flames. This foam also cools the underlying fuel, further aiding in controlling the fire.

The other characteristics listed contribute to various aspects of fire suppression but are not as critical to the primary function of foam. High temperature resistance may be beneficial, but it does not directly influence the foam's ability to suppress fire. Increased density could improve performance in specific scenarios, yet it might not ensure effective coverage over the fuel surface. Finally, while chemical stability over time is important for shelf life and consistency, it does not impact the immediate effectiveness of the foam in suppressing a fire. Therefore, the unique ability to form a blanket is the essential characteristic that makes foam effective in fire suppression.