Here's why:

* Oxidation: In a chemical reaction, oxidation is the loss of electrons.

* Reduction: Reduction is the gain of electrons.

* Oxidizing agent: An oxidizing agent is a substance that causes another substance to be oxidized (lose electrons). It itself gets reduced in the process.

In this reaction:

* Carbon in methane (CH₄) is oxidized. It goes from an oxidation state of -4 in CH₄ to +4 in CO₂. This means it loses electrons.

* Oxygen (O₂) is reduced. It goes from an oxidation state of 0 in O₂ to -2 in CO₂ and H₂O. This means it gains electrons.

Since oxygen is the substance that causes carbon to lose electrons (oxidize), it is the oxidizing agent.