The correct answer is Reactants. Reactants are the substances that undergo chemical change in a chemical reaction. They are the starting materials of the reaction and are typically written on the left side of a chemical equation.

For example, in the combustion of methane, the reactants are methane (CH4) and oxygen (O2). These substances undergo a chemical change to produce carbon dioxide (CO2) and water (H2O), which are the products of the reaction.