The correct answer is C) the principle by which blow down is accomplished. The primary difference between a huddling chamber type safety valve and a nozzle reaction type safety valve is the way the blow down is accomplished. In a huddling chamber type valve, the blow down is achieved through the pressure differential created by the escaping steam, while in a nozzle reaction type valve, the blow down is accomplished through the nozzle reaction force acting on the valve disc. This difference in the blow down mechanism is the key distinction between these two types of safety valves. The other answer choices, while related to safety valve operation, do not directly address the primary operational difference between these two valve types.