What color and symbol indicate that a specific class of fire is prohibited on a fire extinguisher?

The indication that a specific class of fire is prohibited on a fire extinguisher is represented by a black background with a bright red slash. This visual cue is designed to clearly communicate to users that the fire extinguisher is not suitable for the type of fire indicated and should not be used in that situation. The bright red slash effectively grabs attention and serves as a universal warning, helping to prevent improper use that could lead to dangerous outcomes.

In the context of the other options, a red background with a blue circle typically indicates a type of fire extinguisher that is suitable for specific classes of fires, such as electrical or flammable liquids. A yellow background with a green checkmark may represent safety or suitability but does not convey a prohibition. Lastly, a white background with a black "X" might suggest a warning regarding an area or material, but it does not specifically communicate the prohibition of a fire class. Understanding these symbols is crucial for safety in emergency situations involving fires.