Which of the following must be present for combustion to occur?

For combustion to occur, three essential components must be present: air, fuel, and heat. Air provides the necessary oxygen that reacts with the fuel during the combustion process. The fuel serves as the substance that burns, releasing energy. Heat, which acts as an ignition source, is required to raise the temperature of the fuel to a point where it can ignite and sustain the combustion reaction.

This combination of air, fuel, and ignition (heat) is commonly referred to as the "fire triangle" in the study of combustion. Each element is vital; without any one of them, the combustion process cannot initiate or continue.

Other options do not provide a complete set of the necessary components for combustion. For instance, heat and pressure may be involved in specific combustion scenarios but do not constitute the basic requirements by themselves. Likewise, while air and fuel are critical, without heat, combustion cannot commence. Fuel and spark alone also omit the essential role of air in providing oxygen, which is crucial for sustaining a fire. Thus, recognizing the significance of all three components is pivotal in understanding combustion thoroughly.