Final answer:
The reactant side of the equation CH4 + 2O2 → CO2 + 2H2O contains 1 carbon atom, as found in the molecular formula of methane (CH4).
Step-by-step explanation:
The number of carbon atoms on the reactant side of the equation CH4 + 2O2 → CO2 + 2H2O can be determined by looking at the molecular formula of methane (CH4). This molecule contains 1 carbon atom. Since there is only one molecule of methane as a reactant, the total number of carbon atoms on the reactant side is also 1. Therefore, the correct answer to the question is C.1.