The figurative language used in the sentence is personification.
Personification is a literary device where human qualities are given to non-human entities, such as objects or animals. In this sentence, the action of poking at the bag with her finger is given human-like qualities of being fat, which is not a characteristic that can be attributed to a finger. The finger is personified to emphasize the size of the finger and the force with which it is poking the bag.
Therefore, the correct answer is F. Personification.