The correct answer is C. convection. Convection is the heat transfer between a solid surface and a fluid that is in motion relative to the solid. The motion of the fluid is caused by the difference in density between the hot and cold fluid. The hot fluid is less dense than the cold fluid, so it rises. The cold fluid is denser than the hot fluid, so it sinks. This creates a current of fluid that carries heat from the hot object to the cold object.