Answer: B. The energy supplied is $E=Pt$ using the time for which the heater is on: $E=(400\ \text{W})(5.0\times60\ \text{s})=1.2\times10^5\ \text{J}=120\ \text{kJ}$ During melting, the energy changes particle behaviour and arrangement while the average kinetic energy, and therefore the temperature, remains constant.
A: treats 5.0 min as 5.0 s, giving 2.0 kJ.
C: calculates the energy correctly but applies the temperature-rise model instead of recognizing a phase change.
D: uses the full 10.0 min interval as the heater-on time, giving 240 kJ.
Remember: energy can be transferred during a phase change without increasing the temperature.