During combustion, what is required in the flammable range to maintain a fire?

To maintain a fire during combustion, the presence of both oxygen and fuel vapor within the flammable range is crucial. This is because combustion is a chemical reaction that requires three elements known as the fire triangle: heat, fuel, and an oxidizing agent (typically oxygen). In this context, fuel vapor refers to the gaseous form of the material that is burning; without sufficient concentration of fuel vapor along with an adequate amount of oxygen, the fire cannot sustain itself.

The correct answer emphasizes that both oxygen and fuel vapor must be in the right proportions to create a mixture that can ignite and burn continuously. If either element is depleted or not present in sufficient quantities, the combustion process will be interrupted, and the fire will extinguish.

In contrast, choices mentioning heat and pressure, fuel and an ignition source, or carbon and nitrogen do not fully encompass the necessary conditions for combustion as established by the fire triangle concept. While heat and pressure can play roles in the combustion process, they are not sufficient by themselves to maintain a fire without the essential presence of fuel vapor and oxygen. Similarly, while fuel and an ignition source are important for initiating combustion, oxygen is specifically required to sustain the process. Carbon and nitrogen do not serve as essential components for the combustion