Explanation
According to the PMBOK Guide, the project manager is responsible for managing the project work and ensuring that the project objectives are met. If there are issues that affect the projectschedule, scope, quality, or cost, the project manager should escalate them to the appropriate level of authority for resolution. In this case, the supplier-related issues are causing schedule delays on the critical path, which means they could jeopardize the project completion date. Therefore, the project manager should escalate them to the sponsor, who is the person or group that provides the financial resources and support for the project. The sponsor can help resolve the issues by negotiating with the suppliers, providing additional resources, or approving changes to the project plan. References: PMBOK Guide, 7th edition, pages 11, 12, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, and 25.