ANS:C - feed high pressure water to boiler and guard against its backflow through it.

Feed check valves are used in steam boilers to feed high-pressure water to the boiler and guard against its backflow through it. Explanation: Feed check valves, also known as boiler check valves or non-return valves, are essential components in steam boiler systems. Their primary function is to ensure that water flows in one direction only, preventing backflow from the boiler into the feedwater system. This is crucial for maintaining the safety and efficiency of the boiler system. When the boiler is operating, feed water is pumped into the boiler under high pressure to replenish the water lost through steam production. The feed check valve allows this high-pressure feed water to enter the boiler while preventing any backflow of steam or water from the boiler back into the feedwater system. This ensures that the boiler operates safely and efficiently by maintaining the proper water level and preventing damage to the boiler or associated equipment due to reverse flow. Therefore, the correct function of a feed check valve in steam boilers is to feed high-pressure water to the boiler and guard against its backflow through it.