Which type of extinguisher is the least effective against flammable liquid fires?

Water-based extinguishers are the least effective against flammable liquid fires because they can exacerbate the situation rather than control it. Using water on a flammable liquid fire can cause the liquid to spread, as it may not be effectively able to cool or suppress the flames. Water can also vaporize rapidly when it contacts the burning liquid, potentially leading to a greater release of flammable vapors.

In contrast, foam extinguishers are designed to create a barrier over the surface of flammable liquids, preventing oxygen from reaching the fuel and extinguishing the fire effectively. Dry powder extinguishers can smother the flames and interrupt the chemical reaction of fire, making them suitable for various types of flammable materials, including liquids. Carbon dioxide extinguishers work by displacing oxygen in the area of the fire, which is also effective for flammable liquid fires. Thus, while options like foam, dry powder, and carbon dioxide are specifically designed and effective for such incidents, water-based extinguishers pose a risk and are not suitable for handling flammable liquid fires.