Which of the following is NOT a valid form of bid guarantee?

A personal check is not considered a valid form of bid guarantee because it does not provide the immediate assurance of funds that is required in a bidding process. Bid guarantees, such as a cashier's check, certified check, or bid bond, must be reliable and guarantee that the funds will be available if the bidder defaults on their offer.

A cashier's check is guaranteed by the bank and is a secure method of payment. Similarly, a certified check is also verified by the bank, ensuring that the funds are set aside for payment. A bid bond, on the other hand, is a form of surety from a third-party company that ensures the bidder will fulfill their obligations. Each of these options provides a level of security and assurance to the agency inviting bids, which a personal check cannot provide.