Final answer:
For a fire to start, all of the following are required: oxygen, heat, and fuel. Each plays a crucial role in the combustion process.
Step-by-step explanation:
To start a fire, all of the following substances must be present: oxygen, heat, and fuel. Without any one of these, a fire cannot occur. Oxygen in the air supports the chemical processes of combustion by reacting with the fuel. Heat is needed to initiate the combustion reaction and maintain the fire, allowing it to spread.
Lastly, fuel, which is any combustible material, serves as the substance that is consumed by the fire, producing heat and light as it reacts with oxygen.