The announcement of an unexpected bonus led to great tumult among the staff.

'Tumult' means a loud, confused noise or disorder, often with negative connotations. However, since the trigger is an 'unexpected bonus' (a positive event), the appropriate reaction would be joyful, not disorderly. 'Merriment' means cheerful and lively enjoyment, which fits perfectly with staff happily celebrating an unexpected bonus. 'Trepidation' implies fear, which is contextually wrong. 'Upheaval' and 'uproar' both carry negative or disruptive senses, unsuitable for a positive announcement. Therefore, 'merriment' is the correct improvement.