The element that is always a reactant 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.

* The substance being burned is called the fuel, and it can be a solid, liquid, or gas.

* The oxygen acts as the oxidant, combining with the fuel to release energy.

For example, in the combustion of methane (CH4), the chemical equation is:

CH4 + 2O2 → CO2 + 2H2O

Here, methane is the fuel, and oxygen is the oxidant.