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 energy required to break the bonds of reactant molecules
When a chemical reaction takes place, chemical bonds between the reactants must be broken in order for the bonds between product molecules to form. Activation energy is the energy required to break these bonds, and it causes the chemical reaction to start. 

Activation energy is the energy required to break the bonds of reactant molecules. It is the minimum energy required to start off a chemical reaction.