ANS:B - drain out water and sludge during its repair.

The Blow off cock is provided in a steam boiler primarily to drain out water and sludge during its repair. Here's a breakdown of the options:

1. Guard the boiler in case of abnormal rise in steam pressure: This function is typically performed by safety valves, which are designed to release excess steam when the pressure inside the boiler exceeds a predetermined limit. Blow off cocks are not intended for pressure relief.
2. Drain out water and sludge during its repair: This is the primary function of blow off cocks. They are installed at the bottom of the boiler to allow for the drainage of water and sediment (sludge) that may accumulate at the bottom of the boiler during operation. This is important for maintenance and to prevent corrosion and sediment buildup.
3. Regulate the supply of water in the boiler: This function is typically performed by feedwater control valves or regulators, which regulate the flow of water into the boiler to maintain the desired water level. Blow off cocks are not used for this purpose.
4. Put out the fire in the furnace when the water level in the boiler falls to unsafe limit: This function is typically performed by low-water cutoff devices or water level controls, which are designed to shut down the boiler or trigger an alarm when the water level falls below a safe operating limit. Blow off cocks are not designed for fire suppression or water level control.