During what situation should foam be used effectively at an airport?

Foam is particularly effective when extinguishing flammable liquid fires due to its unique properties. Flammable liquids, such as aviation fuels, create hazards that might not respond well to water alone, as water can either float on the surface or exacerbate the situation by spreading the fire. Foam creates a blanket that smothers the fire by separating the fuel from the oxygen in the air, effectively reducing the fire's ability to sustain combustion.

This ability to cover and suppress flammable liquids makes foam the preferred agent for such situations, especially in an airport environment where liquid fuels like Jet A or AVGAS are commonly present. The combination of foam's suppressive qualities and its ability to cool down the surrounding area allows for safer and more efficient fire management. Other scenarios mentioned in the choices do not leverage the specific advantages of foam, which is why using it primarily for flammable liquid fires is the most effective approach.