The project manager should refer to the requirements traceability matrix to verify whether the customer's claims are valid or not. The requirements traceability matrix is a grid that links product requirements from their origin to the deliverables that satisfy them1. It helps to ensure that each requirement adds business value by linking it to the business and project objectives2. It also provides a means to track requirements throughout the project life cycle, helping to ensure that requirements approved in the project scope statement are delivered at the end of the project3. Revising the project scope baseline (B) is not a good option since it would imply accepting the change requests without verifying their validity. Escalating the change request to the project sponsor is also not advisable since the project manager should first analyze the situation and propose a solution. Reviewing the approved project charter (D) is not relevant since the project charter does not contain the detailed product requirements. References:
* 1: A Guide to the Project Management Body of Knowledge (PMBOK Guide) - Seventh Edition, Glossary, page 717
* 2: A Guide to the Project Management Body of Knowledge (PMBOK Guide) - Seventh Edition, Chapter 5, Section 5.2.3.1
* 3: A Guide to the Project Management Body of Knowledge (PMBOK Guide) - Seventh Edition, Chapter 5, Section 5.6.3.1