The correct answer is C) Blow down is the difference between the valve popping pressure and the valve reseating pressure. This is the correct definition of blow down as it pertains to safety valves. Blow down is the pressure drop between the point at which the valve opens (popping pressure) and the point at which the valve reseats and closes (reseating pressure). This pressure drop is an important characteristic of safety valves, as it allows the valve to properly reseat and prevent over-pressurization of the system. The other answer choices are incorrect because they do not accurately define blow down in the context of safety valves. Option A describes a different valve feature, option B defines the difference between maximum allowable working pressure and popping pressure, and option D defines the difference between maximum allowable working pressure and reseating pressure, rather than the pressure drop across the valve opening and closing.