Final answer:
In the reaction CH4 + 2O2, carbon is undergoing oxidation as its oxidation state increases from -4 in methane to +4 in carbon dioxide.
Step-by-step explanation:
To explain which element is undergoing oxidation in the given reaction CH4 + 2O2, we must look at the changes in oxidation numbers. Combustion reactions are a type of redox reaction where oxidation and reduction occur. In the case of methane (CH4), carbon is oxidized as it reacts with oxygen. Considering the initial reaction CH4(g) + 2O2(g) → CO2(g) + 2H2O(g), the carbon in methane has an initial oxidation state of -4, and in carbon dioxide (CO2), it has an oxidation state of +4. Thus, the carbon increases its oxidation state from -4 to +4, and therefore, carbon is the element that is undergoing oxidation in this reaction.