D.No one, no matter how strong, is invulnerable
Explanation:
The load is a story from Aesop's fable and clearly has moral behind all of the small tales that are presented in the book.
This story is about a merchant who makes a donkey carry a load through a particularly rough terrain over and over again and uses the same method when failing to do so.
They do not succeed until it appears to them that using the same method will always come out with the same result.
No matter how much their method has worked for them, no matter how strong it is, it will be vulnerable to fail sometimes.
