The answer is a. An appositive is a noun or noun phrase that renames another noun right beside it. "a city on the Fox River" is renaming Aurora. Therefore, the right answer is a. Good luck.
1. Who 2. Whom 3. Whom 4. Who 5. Whom 6. Who 7. Who 8. Whom 9. Who 10. Whom 11. Who 12. Who 13. Who 14. Who 15. Whom 16. Who 17. Whom 18. Who 19. Who 20. Who 21. Who