Final answer:
The correct option is C. The rate of burning of a liquid pool is measured as the Flame spread rate, expressed in mm/s, indicating the speed of flame movement across the fuel's surface.
Step-by-step explanation:
The term used to express the rate at which a liquid pool burns is Flame spread rate, which is measured in millimeters per second (mm/s). This rate indicates how quickly a flame moves across the surface of a fuel, reflecting the combustion process's intensity and rapidity.
The other terms mentioned have different meanings: Ignition temperature is the minimum temperature needed to ignite a substance, Heat of combustion is the energy released when a substance combusts, and Radiative flux is the rate of radiant energy transfer per unit area.