The answer is A. digestion of food. The chemical change to matter means that the molecule structure has been destroyed. For B, C and D. the matter are still ice, paper and ice. So they are physical change.
The temperature of the substance giving off the heat decreases while the temperature of the substance receilving the heat increases. they leach what is called equlibrium point where heat energy can longer be exchanged hence equql temperature. this isThermal physics
So power is considered as the rate of doing work. Base on the problem given, my analysis is that the machine who finish the work faster is machine C. Therefore, in order to finish the same amount of work in a short period of time you are going to expend the most power. My answer is Machine C.