What type of reaction involves a single compound breaking down into two or more elements or new compounds?

The reaction described pertains to a decomposition reaction. In a decomposition reaction, a single compound undergoes a transformation that results in the breaking apart of that compound into two or more elements or simpler compounds. This process typically requires an input of energy, such as heat, light, or electricity, to initiate the breakdown.

An example of a decomposition reaction is the electrolysis of water, where water (H2O) is separated into its elemental components, hydrogen (H2) and oxygen (O2). This highlights the essence of a decomposition reaction, which is characterized by the division of a complex substance into lesser entities.

Other types of reactions, such as synthesis, exchange, and combination, involve different mechanisms. Synthesis reactions combine two or more substances to form a new compound, exchange reactions involve the swapping of components between compounds, and combination reactions refer specifically to the merging of reactants to create a single product. Therefore, the unique nature of a decomposition reaction, centered on the breakdown of a single compound, confirms the correctness of the chosen answer.