Which of the following substances is NOT required for a fire to occur?

For a fire to occur, three essential elements are required, often referred to as the fire triangle: fuel, heat, and oxygen. Fuel is the material that burns, heat is the energy that raises the material to its ignition temperature, and oxygen is necessary to sustain combustion.

Water is not a necessary element for fire to occur. In fact, it is often used to extinguish fires because it cools the fuel and reduces the heat component of the fire triangle. While water can help in firefighting efforts, it does not play a role in the initial conditions needed to start a fire. Hence, the absence of water does not prevent combustion from happening, making it the correct answer in this question.