The ice at -20°C converts to ice at 0°C, thus having an increase in temperature. On further heating, ice at 0°C is converted into water at 0°C, thus the temperature remaining constant, a change of phase occurs. Water at 0°C when heated changes to water at 100°C, thus resulting in an increase of temperature. Water at 100°C changes to steam at 100 °C, thus resulting in a change of phase. Hence, Fig. (a) is the correct answer.