Which of the following is NOT a type of fire extinguisher?

The selection of foam as the answer gives insight into the classification of fire extinguishers. In fire safety, foam is not categorized as a distinct type of fire extinguisher; rather, it is a type of extinguishing agent used within certain types of fire extinguishers. Specifically, foam extinguishers are designed to combat flammable liquids and are commonly found in the class B and class A categories.

In contrast, stored pressure and cartridge pressure refer to specific methods of propellant use in extinguishers. Stored pressure extinguishers keep the extinguishing agent under constant pressure, while cartridge pressure extinguishers utilize a cartridge to create pressure when activated. The term "pump" generally refers to a type of extinguisher that is manually operated, typically used in smaller models meant for quick accessibility.

Therefore, while foam is a type of agent featured in some fire extinguishers, it does not constitute a category of fire extinguisher by itself, making it the correct answer to the question.