D. liquid water vaporizes.
I have attached a phase diagram for water. As you can see, if kPa is 50, and the temperature is 100c. it is in the vapor phase.
When ice cream melts from solid to liquid, the motion of the molecules increases. This is because as the phase change moves from solid to liquid to gas, entropy increases which increases the probability of the molecules to collide and move in the system. This increase may be because of the increase in temp, probable cause of the melting of icecream.