Answer: the activation energy.
Explanation:
1) The graph attach is the energy diagram of a chemical reaction: it shows the changes in the energy of the substances (reactants and products) as the reaction progress.
2) The flat section A represents the energy of the reactants (the substances before a chemical reation happens).
3) The peak B represents the energy of the activated complex: this is the intermediate compound that is formed as a necessary step in the conversion process from reactants to products.
4) Therefore, the arrow identified with the letter D is the difference of energy of the activated complex and the reactants. This is the energy that the reactants have to absorb to accomplish the conversion. This enery is seen as a barrier and is named activation energy.
The flat section C represents the energy of the products. In this case the products are lower in energy than the reactants, and so there is a net release of energy: it is an exothermic reaction.
