Endothermic reactions are characterized by the absorption of heat, typically involving bond breaking processes. In contrast, exothermic reactions release heat and often involve bond formation. To identify an endothermic reaction, it is essential to analyze the phase changes and the nature of the bonds involved.

For instance, when steam condenses, it transitions from a gas to a liquid. This process involves the formation of bonds rather than breaking them, thus it is not endothermic. Similarly, when molten lava solidifies, it changes from a liquid to a solid, again forming bonds and not breaking them, which disqualifies it as an endothermic reaction.

On the other hand, when water boils, it undergoes a phase change from liquid to gas. This process requires energy input to break the intermolecular bonds in the liquid, making it an endothermic reaction. Lastly, water freezing involves a transition from liquid to solid, where bonds are formed, indicating that this process is also not endothermic.

In summary, among the processes discussed, only the boiling of water represents an endothermic reaction, as it involves the breaking of bonds and the absorption of heat.