Which acquisition method requires a contractor to begin work without a fixed-price?

The acquisition method that requires a contractor to begin work without a fixed price is known as a "cost-reimbursement contract." In this arrangement, the contractor is paid for allowable expenses incurred during the project, along with an additional amount for profit. This method is often used when the scope of work is uncertain, making it difficult to establish a fixed price upfront. It allows for flexibility and adaptability as the project progresses.