The correct answer is C) high steam pressure. The solenoid valves in the fuel oil supply line to an automatically fired auxiliary boiler are designed to automatically close when the steam pressure reaches a high level. This is a safety feature to prevent overpressurization of the boiler system in the event of a malfunction or other issue that could lead to a dangerous increase in steam pressure. The high steam pressure triggers the closure of the solenoid valves, shutting off the fuel supply and helping to prevent a potentially hazardous situation. The other options are incorrect because they do not directly relate to the automatic closure of the solenoid valves. A decrease in feed temperature, high furnace air pressure, or low steam pressure would not directly trigger the solenoid valve closure in the same way that high steam pressure does.