A developing fire must have sufficient _____ to reach flashover.

To understand why the correct answer is oxygen, it’s important to recognize that the flashover phenomenon in fire behavior occurs when a room or area reaches a temperature that ignites all combustible materials simultaneously. For this dramatic transition to happen, a fire must be adequately supplied with oxygen, as it is essential for combustion processes.

In a fire, oxygen is one of the three critical components of the fire triangle, which also includes heat and fuel. As the fire develops, the surrounding environment needs to provide enough oxygen for the combustion process to escalate to the point of flashover. Without sufficient oxygen, the flames may smolder or diminish instead of intensifying toward flashover.

While fuel, heat, and even hydrogen, which is sometimes present in the environment of a fire, can contribute to fire behavior and intensity, it is the availability of oxygen that directly influences the ability of a fire to reach flashover. Therefore, an oxygen-deficient environment will hinder the fire's development and prevent it from reaching the critical transitional stage of flashover.