The correct answer is option 'C', Economiser.

Explanation:
A boiler mounting refers to various devices and components that are attached to a boiler for its proper functioning and safety. These mountings ensure the efficient operation of the boiler and help in maintaining its overall performance. Let's discuss each of the given options to understand why economiser is not considered a boiler mounting:

a) Blow off cock:
A blow off cock is a valve that is used to remove sediments and impurities from the bottom of the boiler. It allows the discharge of water and other impurities from the boiler to maintain its water quality. This helps in preventing the formation of scale and maintaining the efficiency of the boiler.

b) Feed check valve:
A feed check valve is a device that is used to control the flow of water into the boiler. It ensures that water flows only in one direction, preventing the backflow of water from the boiler. This valve helps in maintaining a constant water level inside the boiler and ensures continuous supply of water for steam generation.

c) Economiser:
An economiser is a heat exchanger device that is used to recover heat from the flue gases of a boiler. It is typically installed in the flue gas path before the chimney. The purpose of an economiser is to preheat the feedwater entering the boiler using the waste heat from the flue gases. This helps in improving the overall efficiency of the boiler by reducing fuel consumption.

d) Fusible plug:
A fusible plug is a safety device that is installed in the crown sheet of the boiler. It consists of a brass plug containing a fusible alloy. The fusible plug is designed to melt and release steam when the temperature of the crown sheet exceeds the safe limit. This helps in preventing the boiler from getting overheated and avoids the risk of a boiler explosion.

In summary, all the given options (blow off cock, feed check valve, and fusible plug) are considered boiler mountings as they are directly attached to the boiler and play a crucial role in its operation and safety. However, an economiser is not considered a boiler mounting as it is not directly attached to the boiler but rather a separate heat exchanger device that enhances the boiler's efficiency.