The reaction of hydrogen (H2) and oxygen (O2) to produce water vapor (H2O) is a combustion reaction.

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.

* In this case, hydrogen acts as the fuel, and oxygen acts as the oxidant.

* The reaction releases a significant amount of energy in the form of heat and light, which is why this reaction is often used for generating energy.

The balanced chemical equation for this reaction is:

2H2 + O2 → 2H2O