The correct answer is option 'C' - Fusible plug.

A boiler is a closed vessel that is used to generate steam by heating water. It is important to maintain the water level in the boiler at a safe level to prevent any accidents or damage. If the water level falls below the safe level, the intense heat from the furnace can cause damage to the boiler.

A fusible plug is a fitting that is used to extinguish the boiler furnace fire in case the water level falls below the safe level. It is located at the crown sheet of the furnace, which is the topmost part of the boiler. The fusible plug is made of a low melting point alloy, such as lead or a lead alloy.

When the water level falls below the safe level, the intense heat from the furnace causes the alloy in the fusible plug to melt. This creates an opening in the plug, allowing water to flow into the furnace and extinguish the fire. The flow of water also helps to cool down the crown sheet and prevent any damage to the boiler.

The fusible plug acts as a safety device and is designed to melt at a specific temperature, which is lower than the melting point of the boiler material. This ensures that the plug will melt and extinguish the fire before any damage occurs to the boiler.

It is important to regularly inspect and replace the fusible plug to ensure its effectiveness. The plug should be replaced if it shows signs of corrosion or if it has been melted in the past. Regular maintenance and inspection of the fusible plug can help prevent accidents and ensure the safe operation of the boiler.

In conclusion, the fusible plug is used to extinguish the boiler furnace fire in case the water level falls below the safe level. It is a safety device that melts at a specific temperature, allowing water to flow into the furnace and extinguish the fire. Regular maintenance and inspection of the fusible plug are essential to ensure its effectiveness.