Final answer:
The fire triangle includes an ignition source, oxygen, and fuel. These three elements are crucial to initiate and sustain a fire, and are not to be confused with firefighting tools like a fire extinguisher.
Step-by-step explanation:
The fire triangle includes three elements necessary for a fire to occur: 1) an ignition source, 2) oxygen, and 4) fuel. The third option listed, a fire extinguisher, is not part of the fire triangle but rather a tool used to remove one of these elements and thus extinguish the fire. The importance of this concept is that in order for a fire to start and sustain itself, it requires a source of heat to ignite the fuel, a sufficient amount of oxygen to maintain the combustion, and a fuel to keep the fire burning. This corresponds to the physical and chemical properties discussed in science curriculum when studying combustion reactions.