The answer is A) Whimsical
Whimsical can be described a feeling of 'fanciful, playful and mischievous'
A lot of people think 'Nostalgia' is the correct answer but it is not. If the novel was set during summer vacations of a school and described a child's life during those months, it might have been trying to develop a sense of nostalgia.
However, in this case, a simple addition of the amusement part, slightly changes the tone of the novel.