Which element in air is crucial for the combustion process?

Oxygen is the essential element in air that facilitates the combustion process. Combustion is a chemical reaction in which a fuel reacts with oxygen to produce heat, light, and byproducts such as carbon dioxide and water. In a combustion reaction, the presence of oxygen is necessary for the fuel to ignite and maintain the reaction.

While nitrogen, hydrogen, and carbon dioxide are present in air, they do not support combustion. Nitrogen is primarily an inert gas in this context and does not contribute to the combustion process. Hydrogen can burn when sufficient oxygen is present, but it is not a component necessary for combustion to occur; rather, it is a fuel that can combust. Carbon dioxide, on the other hand, is a product of combustion and acts to inhibit the process rather than support it.

Therefore, oxygen is crucial for combustion, making it the correct answer to the question.