The gas present in air that takes part in a combustion reaction is oxygen (O₂).

Here's why:

* Combustion is a chemical process that involves the rapid reaction between a substance with an oxidant, usually oxygen, to produce heat and light.

* Oxygen acts as the oxidizer, combining with the fuel source (like wood, gasoline, or natural gas) to release energy in the form of heat and light.

While air is primarily composed of nitrogen (around 78%), nitrogen itself does not participate in combustion.