Answer: Option (D) is the correct answer.
Step-by-step explanation:
It is known that in a closed system there will no exchange of heat or matter from the system to the surrounding. Whereas in an open system there will be exchange of both heat and energy from the system to the surrounding and vice versa.
Therefore, in the given situation cup A is a closed system and cup B is an open system. Hence, there will be no exchange of heat or matter from cup A to system and surrounding. So, it will cool slowly that cup B.
Thus, we can conclude that the statement no molecules will be able to evaporate at the surface of cup A, best explains why cup A will cool more slowly than cup B.