The energy of activation for a reaction is best defined as:

The energy of activation, or activation energy, is defined as the minimum energy required for a reaction to occur. This is represented by the energy difference between the activated complex (the transition state at the highest energy point during the reaction) and the energy of the reactants.

In this context, when you consider the reaction pathway, the activated complex must be formed before the reactants can convert to products. Therefore, the energy required to reach this activated complex from the energy level of the reactants characterizes the activation energy.

Thus, the correct description aligns with the difference in energy between the activated complex and the reactants, highlighting that the activation energy defines the barrier that must be overcome for a reaction to proceed.