Which reaction occurs involving ionic compounds where the reactivity determines the action taken?

The correct choice involves understanding the nature of ionic compounds and their reactivity. In replacement reactions, especially involving ionic compounds, one element displaces another in a compound based on their reactivity. This often occurs when metals or halogens are involved, as their position in the activity series dictates whether a displacement will happen.

In a typical single replacement reaction, a more reactive element displaces a less reactive one from its compound. For example, when zinc is added to copper sulfate, zinc displaces copper due to its higher reactivity, resulting in the formation of zinc sulfate and copper metal. This process is a clear illustration of reactivity determining the action taken during the reaction.

The other types of reactions listed do not inherently depend on the reactivity of the compounds involved in the same manner. Decomposition reactions involve breaking down compounds into simpler substances, combustion reactions deal with substances reacting with oxygen to produce energy, and while replacement reactions can occur, they are typically categorized under single or double replacement specifically. The emphasis on reactivity in the context given makes replacement reactions the most appropriate response.