A manager prepared and signed checks payable to a fictitious supplier and depositted the checks into their personal bank account. Which of the following internal controls most likely have prevented, or at least detected, the embezzlement?

a. Payments to suppliers must be made with a certified check.

b. A check signer, other than the manager, must sign checks when that manager is responsible for approving bills that are presented for payment.

c. Competitive bids are used for all purchases.

d. The numerical sequence of checks must be accounted for by a responsible employee on a regular basis.