Answer: option d. the intermolecular forces of the substance get stronger.
Justification:
1) The heat of vaporization is the heat required by a substance to overcome the intermolecular forces in the liquid state and pass to gas state.
2) From that, you can directly predict that the stronger the intermolecular forces the higher the heat of vaporization.
3) Graphycally that will be translated in a longer segment: the horizontal axis is time and the greater the heat of vaporization the longer the time needed.