The best definition of activation energy is (a) the energy required to break the bonds of reactant molecules.
Activation energy is a crucial concept in chemistry that refers to the minimum amount of energy needed for a chemical reaction to occur. When reactant molecules come together, they must overcome a certain energy barrier to break the bonds holding them together, which is where activation energy comes into play. Without enough energy, the reaction won’t proceed, regardless of the concentrations of the reactants.
Option (b) refers to the energy needed to terminate a reaction, which is not the same as activation energy. Option (c) suggests energy for binding a substrate to an active site, which relates more to enzyme activity than to the definition of activation energy itself. Finally, option (d) is incomplete and does not provide a valid choice. Therefore, option (a) clearly captures the essence of activation energy within the context of chemical reactions.