Which of the following is a necessary ingredient for a fire?

A necessary ingredient for a fire is fuel. Fuel is any material, such as wood, paper, gasoline, or other combustible substances, that can be used to sustain a fire. In combination with oxygen and heat, fuel allows the chemical reaction of combustion to occur, which produces flames. Without sufficient fuel, a fire cannot be maintained, as there would not be the necessary material to continue the reaction.

The other options do not serve as essential components for fire. Carbon dioxide is a product of combustion and can actually help extinguish fires by displacing oxygen. Water is often used to put out fires rather than sustain them, as it cools the burning material and removes heat. A vacuum lacks oxygen altogether, meaning it cannot support combustion at all. Therefore, fuel is the critical element that establishes and keeps a fire burning.