Which best describes what is occurring in the reaction?

Actually, The reaction is chemical and endothermic. Chemicals A and B form an endothermic reaction, and chemicals C and D form an exothermic reaction. Therefore, chemical reaction, a process in which one or more substances, the reactants, are converted to one or more different substances, the products. Substances are either chemical elements or compounds. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.

Bond breakage and reformation.

Explanation:

During chemical reactions, atoms within the participating reactants are rearranged. These rearrangement occur as a result of breaking and reforming of bonds.

When the bond holding the atoms within a particular reactant is broken, it cannot reform except if the reaction is a reversible one. New bond are formed between atoms of different reactants.