Which type of foam produces a fast-spreading film with a surface-active agent?

The correct answer highlights aqueous film-forming foam (AFFF) as the type of foam that produces a fast-spreading film with a surface-active agent. AFFF is specifically designed to provide rapid coverage over fuels, creating a thin film that separates the fuel from the oxygen in the air, thereby suppressing flammable vapors.

This foam utilizes a combination of surfactants that reduce surface tension, enabling the foam to quickly spread across the surface of liquids and create a protective barrier. The ability to form this film is crucial in fighting flammable liquid fires, such as those involving hydrocarbons, where quick control of vapors is essential.

In contrast, Class A foam is primarily used for enhancing water's effectiveness on ordinary combustibles and does not primarily produce a fast-spreading film for flammable liquids. High expansion foam is characterized by its ability to create a large volume of foam that expands significantly, but does not create the thin film associated with AFFF. Fluoroprotein foam is designed for similar applications to AFFF but does not produce the same fast-spreading film due to its thicker consistency.

By understanding the specific properties and intended applications of these foams, responders can select the most effective type for managing various fire scenarios.