The combustion reaction is also called an oxidation reaction. Oxidation refers to the reaction between a substance being oxidized and oxygen. Combustion will always form CO2 and water as products.
Now, when a substance is oxidized it is because it loses electrons, the cause of this loss of electrons is oxygen. Now, the oxygen oxidizes the entire fuel molecule, that is, it attacks both carbon and hydrogen.
Both elements oxidize, carbon forms CO2 and hydrogen forms H2O, so it could be said, if it is possible to choose multiple options, that the elements that oxidize are carbon and hydrogen.
If you can only choose one option, priority will be given to carbon, which is itself the one that generates the greatest amount of energy when oxidized.
As you can only choose one option the answer will be: Carbon is oxidized