Concept:
Latent Heat of Vaporization: It is defined as the amount of heat required to change the state of mater without changing of its temperature.
From the given question, the temperature at the boiling point remained constant despite the continued addition of heat by the Bunsen burner. Actually, this amount of heat is used by water to break the intermolecular bonds between the water molecules in the form of latent heat that converts the liquid state of water into vapor state of water.
Hence, the correct option will be d.The energy was used to break the intermolecular bonds between the water molecules.