Which of the following is an example of a decomposition reaction?

A decomposition reaction is characterized by a single compound breaking down into two or more simpler products. In the given example, the reaction AB → A + B represents a single compound, AB, that breaks apart into its individual components, A and B. This process is indicative of decomposition because it involves the separation of a compound into its constituent elements or simpler compounds.

In contrast, the other options illustrate different types of reactions. For instance, the first choice reflects a combination or synthesis reaction, where two reactants combine to form a single product. The second choice is representative of a combination reaction as well, where two reactants form a compound. Lastly, the fourth option involves a single displacement reaction, where an element replaces another element in a compound. Each of these differ in mechanism and outcome from the decomposition reaction highlighted in the correct answer.