Introduction:
The burning of fire is a chemical reaction known as combustion. It occurs when a substance combines with oxygen to release heat and light. In order for fire to burn, it requires three essential elements: fuel, heat, and oxygen. Without any one of these elements, fire cannot sustain itself.

Explanation:
1. Oxygen:
Oxygen is the most crucial element required for fire to burn. It is present in the air we breathe, making up approximately 21% of the Earth's atmosphere. During the process of combustion, oxygen molecules combine with the fuel to produce heat and light. Without oxygen, the fire cannot undergo the necessary chemical reactions to sustain itself.

2. Fuel:
Fuel is any material that can undergo combustion. It can be in the form of solid, liquid, or gas. Common examples of fuel include wood, coal, gasoline, and natural gas. When fuel is heated, it releases flammable gases that can react with oxygen to produce fire. The fuel provides the carbon and hydrogen atoms necessary for the combustion process.

3. Heat:
Heat is the energy required to initiate and sustain the combustion process. It is usually provided by an external source, such as a spark, flame, or friction. Once the fuel is heated to its ignition temperature, it starts releasing flammable gases. These gases then combine with oxygen in the presence of heat to produce a flame.

Role of other elements:
While oxygen is the primary element required for fire, the other elements mentioned in the options (carbon dioxide, nitrogen, and water vapor) do not play a direct role in the burning process.

- Carbon dioxide (CO2) is produced as a byproduct of combustion. It is released into the atmosphere during the burning of fuel. However, it does not contribute to the fire's ability to burn.

- Nitrogen (N2) is the most abundant gas in the Earth's atmosphere, making up approximately 78%. Although it is present during the combustion process, it does not actively participate in the burning of fire.

- Water vapor (H2O) is formed when the fuel contains hydrogen atoms. It is a product of combustion and can be seen as steam or smoke during a fire. However, water vapor does not support or sustain the fire's burning process.

Conclusion:
In conclusion, the burning of fire requires oxygen as the essential element. Oxygen molecules combine with fuel in the presence of heat to produce the chemical reactions necessary for combustion. While other elements may be present during the burning process, they do not directly contribute to the fire's ability to burn.