Why should the melting ice be used?
Melting ice is used to make sure that the temperature of the ice is at 0度. Hence, no energy is absorbed as the heat capacity to raise the temperature of the ice to 0度.
Why should the crushed ice be used?
Crushed ice should be used. This enhances the thermal contact between the ice and heater. Therfore, less heat is wasted to heat up the air between the ice cubes.