Final answer:
Safety valves used on a super-heater discharging steam over 450 degrees F should be made of steel alloy because it can withstand high temperatures and pressures, ensuring safety and durability. The correct answer is D.
Step-by-step explanation:
The question asks about the appropriate material choice for safety valves used on a super-heater discharging super-heated steam at a temperature over 450 degrees Fahrenheit. In such high-temperature applications, the material of the valve must be able to withstand not only the thermal stress but also the pressure and corrosive nature of the steam. Considering these conditions, steel alloy is typically used for these valves. Unlike brass or cast iron, steel alloys are known for their high temperature and pressure resistance, making them suitable for applications involving superheated steam.
Materials such as brass (option a) and cast iron (option b) lack the necessary properties for such high-temperature environments, where malleable iron (option c) might offer some resistance but still not as good as steel alloys. Hence, steel alloys are the preferred choice because they can ensure the safety and durability required for the valves to function properly even in the event of a loss of coolant accident, as auxiliary cooling systems might also be part of the safety features for reactors.