In these lines, the theme of the story is:
B. A happy life can turn into a sorrowful one in a moment's notice.
These lines are from the story “A Monk's Tale” by Geoffrey Chaucer. The monk of the story was of the view that a story should be narrated so as to encourage people to live a religious life. He regards fortune as whimsical because it possesses the power to change anyone's life according to the deeds done by the individual.