The correct option is A boiling point
If we take some amount of water and start heating it, there will be a stage when the temperature in the thermometer will remain constant.
This happens because all the heat provided at that time is utilised by liquid water particles to break the forces of attraction, move apart and change into steam (gaseous form of water).
This temperature is known as the boiling point of water.
The boiling point of pure water is 100 0C or 373 K at 1 atm pressure.