Which of the following defenses would best serve Bill's?
By depositing the check without protest and with knowledge of its wording, Tune discharged any remaining duty to pay on the part of Bill's. 
By depositing the check without protest and with knowledge of its wording, Tune entered into a novation discharging any remaining duty to pay on the part of Bill's. 
The parties' good-faith dispute over return of the radios suspended the duty of Bill's, if any, to pay any balance due.
Tune's deposit of the check and its return to Bill's after payment estopped Tune thereafter to assert that Bill's owed any additional amount.