The most effective extinguishing device known for dealing with Class A and B fires is:

Water fog is considered the most effective extinguishing device for Class A and B fires because it can effectively cool down burning materials and reduce the availability of oxygen necessary for combustion. Class A fires, typically involving solid combustibles such as wood and paper, can be suppressed by the cooling effect of water, which lowers the temperature of the burning material below its ignition point.

For Class B fires, which involve flammable liquids and gases, the use of water fog can help to cool the surrounding area and prevent the fire from spreading. The fine mist created by water fog can also displace some of the oxygen around a fire, further inhibiting combustion and effectively controlling the fire.

While other extinguishing agents mentioned in the choices may be suitable for specific fire types, water fog’s dual capability to cool and suppress makes it a versatile and highly effective choice for both Class A and B fires. This makes it the preferred option in many firefighting scenarios involving these classes.