Answer: a. to emphasize the power of Orpheus music.
In this excerpt, the author uses hyperbole. Hyperbole is a rhetorical device that consists of a gross exaggeration. The author of this text uses this hyperbole when he says that the music Orpheus played was so harmonious it would make the rivers stop in order to listen to it. The most likely reason why the author exaggerates in this way is in order to show how powerful and beautiful Orpheus music was.