A boiler mounting is a device or equipment that is installed on a boiler to enhance its performance, efficiency, or safety. It helps in the proper functioning of the boiler system. Among the options given, the correct answer is option 'D', which is the blow down cock. Here is an explanation of the different options and why the blow down cock is considered a boiler mounting:

Superheater:
- A superheater is a device that is used to increase the temperature of the steam generated in a boiler.
- It is typically installed in the convective or radiant zone of the boiler.
- Its purpose is to remove any remaining moisture in the steam and increase its thermal energy content.
- The superheater helps in achieving higher steam temperatures, which can be useful in certain industrial processes.
- However, it is not considered a boiler mounting but rather a boiler accessory.

Economizer:
- An economizer is a heat exchanger that is used to recover heat from the flue gases of a boiler.
- It is typically installed in the exhaust gas stream after the boiler.
- The economizer preheats the feedwater entering the boiler, thereby reducing fuel consumption and increasing efficiency.
- It is an important component for energy conservation in a boiler system.
- However, it is not considered a boiler mounting but rather a heat recovery device.

Air pre-heater:
- An air pre-heater is a device that is used to heat the combustion air before it enters the boiler.
- It is typically installed between the economizer and the boiler.
- The air pre-heater utilizes waste heat from the flue gases to preheat the combustion air, resulting in improved combustion efficiency.
- It helps in reducing fuel consumption and emissions.
- However, it is not considered a boiler mounting but rather an energy-saving device.

Blow down cock:
- A blow down cock is a valve or fitting that is used to remove impurities or sediments from the boiler water.
- It is typically installed at the bottom of the boiler shell.
- The blow down cock allows for the periodic removal of sludge, scale, or other solid particles that accumulate in the boiler.
- This helps in maintaining the water quality and preventing the formation of deposits that can reduce the efficiency and lifespan of the boiler.
- The blow down cock is an essential component of the boiler system and is considered a boiler mounting.

In summary, while options 'A', 'B', and 'C' are important components in a boiler system, they are not considered boiler mountings. The blow down cock, on the other hand, is a fitting specifically designed to enhance the performance and safety of the boiler and is therefore classified as a boiler mounting.