During which of the following reactions does an exchange of ions occur?

An exchange of ions is characteristic of a double-displacement reaction, where two compounds react and the ions of each compound switch places to form two new compounds. This type of reaction typically occurs in aqueous solutions where ionic compounds are soluble and can break apart into their constituent ions.

In a double-displacement reaction, the general form is AB + CD → AD + CB, where A and C are cations and B and D are anions. For instance, when silver nitrate reacts with sodium chloride, the silver ions exchange with the sodium ions, resulting in the formation of silver chloride and sodium nitrate. This ion exchange is crucial for processes like precipitation reactions, where an insoluble product is formed.

In contrast, synthesis reactions involve combining two or more simple substances to form a more complex compound, while decomposition reactions break down a compound into simpler products. Combustion reactions involve a substance reacting with oxygen, typically producing heat and light, without an exchange of ions. Therefore, the nature of double-displacement reactions and their inherent ion-swapping mechanism makes them the correct answer in this context.