When carbon reacts with oxygen to form carbon dioxide, it 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, carbon (C) reacts with oxygen (O2) to produce carbon dioxide (CO2).

* The reaction releases heat and light, making it a combustion reaction.

The chemical equation for this reaction is:

C + O2 → CO2