Boiler mountings are the essential components that are installed on a boiler for its safe and efficient operation. They are designed to control various operations and ensure the safe functioning of the boiler. Among the given options, the correct answer is option 'C' - Economiser.

An economiser is not a boiler mounting but rather a boiler accessory. It is a heat exchanger that is typically installed in the flue gas path of a boiler. Its primary function is to recover waste heat from the flue gases and transfer it to the incoming feedwater. This helps in improving the overall boiler efficiency by preheating the feedwater before it enters the boiler.

On the other hand, the other options listed (a, b, and d) are all boiler mountings:

a) Blow off cock: It is a valve located at the bottom of the boiler shell or drum. Its purpose is to discharge the sediment and impurities that accumulate at the bottom of the boiler. This helps in maintaining the water quality and preventing the formation of scale or deposits.

b) Feed check valve: It is a non-return valve that is installed in the feedwater line. Its function is to allow the flow of feedwater into the boiler while preventing backflow from the boiler. This ensures a continuous and controlled supply of water to the boiler.

d) Fusible plug: It is a safety device that is installed in the crown sheet of the firebox or combustion chamber of a boiler. It consists of a hollow brass plug filled with a low melting point alloy. In case of a low water level condition, the plug melts and allows the passage of steam into the combustion chamber. This helps in cooling down the metal surfaces and preventing a dangerous build-up of pressure.

In summary, while blow off cock, feed check valve, and fusible plug are all boiler mountings that serve specific purposes for safe boiler operation, the economiser is a boiler accessory that helps in improving boiler efficiency by recovering waste heat.

Because economizer is boiler accessory and others are mounting