The heating surface of a boiler consists of all parts that:

The heating surface of a boiler is defined by the areas where heat is transferred to the water within the boiler. The correct choice emphasizes the interaction between water and the hot gases produced during combustion.

In this scenario, the heating surface specifically includes parts that have water on one side and hot gases on the other side. This configuration facilitates efficient heat transfer, as the hot gases heat the water, causing it to turn into steam. It is crucial for the heating surface to allow for maximum contact and heat exchange between these two contrasting phases (water and hot gases) to ensure effective boiler operation and steam generation.

The other options do not accurately describe the definition of a heating surface. For instance, having steam on one side and hot gases on the other would indicate that heat is being transferred from the gases to pre-existing steam rather than to the water. This could lead to inefficiencies in the heating process. Similarly, areas with water and steam or those merely exposed to hot gases do not encompass the primary function of a heating surface, which is to transfer heat directly to the water to facilitate steam production.

Thus, identifying the correct interaction of water and hot gases is essential for understanding the mechanism of heat transfer in a boiler system.