The source of the allusion sour grapes is a fable.
In the fable "The Fox and the Grapes," a fox tries unsuccessfully to reach a bunch of grapes. Because he cannot reach them and therefore cannot eat them, he tells himself that they must be sour.
The allusion sour grapes means criticizing something because you can't have it.
What is the source of the allusion in the sentence below?
Vince's remark about your new car is clearly a case of sour grapes.
A: a fable
B: Greek mythology