Final answer:
Cassie is the assignor as she transferred her right to receive the $10 from Emilia to Kelsey for the payment of the party bus, making Kelsey the assignee.
Step-by-step explanation:
In the scenario provided, we are dealing with a debt assignment situation, which falls under the subject of law. The assignor is the entity or individual who transfers the rights and obligations of a debt to another party. Initially, Emilia owed Cassie $10. However, when Kelsey paid $20 for both her and Cassie's ride, Cassie directed Emilia to pay the $10 to Kelsey instead. In this case, Cassie acts as the assignor, because she is transferring her right to receive $10 from Emilia to Kelsey. Kelsey, in return, becomes the assignee, now holding the right to receive the payment from Emilia that was originally owed to Cassie.