A procurement audit is a structured review of the procurement process and the contracts that were established with the suppliers. It evaluates the performance of the suppliers, the compliance with the contract terms and conditions, the quality of the deliverables, and the effectiveness of the procurement management. A procurement audit can help the project manager understand why there is an unexpected financial balance remaining at the end of the project, by identifying any errors, discrepancies, overpayments, or underpayments that occurred during the procurement process. A procurement audit can also provide recommendations for future improvements and lessons learned. The other options are not the best ways to understand why there is an unexpected financial balance remaining, because they may not focus on the procurement aspect of the project, or they may not provide enough detail or analysis to explain the cause of the issue. References: Project Procurement Management - PMBOK Guide, Procurement Audit - Project Management Knowledge