The general theme of "The Monk's Tale" is the ephemerality of luck or success. Through the tragic stories it tells from various outstanding characters throughout history and in mythology, we can understand why it suggests "Let no man trust in blind prosperity" (sooner or later failure will come) . The following excerpt supports the answer.
"I will bewail, in manner of tragedy,
The harm of them that stood in high degree,
And felle so, that there was no remedy
To bring them out of their adversity.
For, certain, when that Fortune list to flee,
There may no man the course of her wheel hold:
Let no man trust in blind prosperity;
Beware by these examples true and old."
So, the answer presenting the dominant moral of the text is:
"C. For sure it is, if Fortune wills to flee, / No man may stay her course or keep his hold"
