Which is the best definition of activation energy?
the energy required to end a chemical reaction
the energy required to bind a substrate to an active site
the energy required to break the bonds of reactant molecules
the energy required to re-form bonds in product molecules

The correct answer is Energy required to break the bonds of reactant molecules (to start a reaction) is the activation energy. It is the minimum amount of energy necessary for a reaction and it was defined as such as far back as the 19th century, when a Swedish scientist discovered it.