Correct Answer is: (b) Pt/M
For thermal equilibrium in the molten state, the rate of absorbing heat = P = the rate of losing heat due to radiation, etc.
When the power source is turned off, the system continues to lose heat at the same rate, as its temperature does not change.
Therefore, heat lost in time t = Pt = required heat loss for complete solidification = ML, where L = specific latent heat of fusion.
