ANS:B - evaporate the water.

To change 1 kilogram of water at 0°C to superheated steam at 300°C and 1 atm pressure, several steps are involved, each requiring heat energy:

1. Heating the water from 0°C to 100°C: This step involves raising the temperature of water from its initial temperature (0°C) to its boiling point at atmospheric pressure (100°C). The amount of heat required for this step is given by the specific heat capacity of water.
2. Evaporating the water: Once the water reaches its boiling point (100°C), additional heat energy is needed to overcome the latent heat of vaporization and convert the water into steam at the same temperature. This step involves a phase change from liquid to vapor without a change in temperature.
3. Superheating the steam: After all the water has been converted into steam, further heat energy is required to increase the temperature of the steam from its boiling point (100°C) to the desired superheated temperature (300°C). This step involves heating the steam above its saturation temperature while maintaining it in the gaseous state.

Among these steps, the major heat consumption in the process will be evaporating the water (step 2). This is because the latent heat of vaporization required to convert water from liquid to vapor is significantly higher than the heat required to raise the temperature of water or to superheat the steam. Therefore, the correct answer is to evaporate the water.