Answer:
C. This summary is weak because it leaves out the exposition of the story.
Explanation:
The exposition of a story contains the background information about the setting, characters, prior plot events, historical context, etc.
This summary practically begins from the middle of Cinderella, from the moment she goes to the ball. There is nothing about how, after her mother's death, her father remarried her evil stepmother, about how she and her two daughters torment her. From this summary, we wouldn't be able to conclude why Cinderella needs the help of a fairy godmother and why the slipper is such a big deal. That's why the missing exposition makes this summary weak.