The project manager should next review the quality management plan. The quality management plan is a component of the project management plan that describes how the project team will implement the quality policy and satisfy the quality requirements for the project and its deliverables1. It includes the quality standards, quality objectives, quality roles and responsibilities, quality tools and techniques, quality metrics, quality audits, and quality control and validation activities1. By reviewing the quality management plan, the project manager can identify the root cause of the quality issues, determine the corrective and preventive actions, and ensure the compliance with the quality expectations of the key stakeholder.

Option A is not the best answer because reviewing the stakeholder engagement plan is not the next thing to do. The stakeholder engagement plan is a component of the project management plan that identifies the strategies and actions required to promote productive involvement of stakeholders in project decisions and execution2. It is important to review and update the stakeholder engagement plan throughout the project, but it is not the priority when the deliverables are not functioning as expected.

Option C is not the best answer because reviewing the requirements traceability matrix is not the next thing to do. The requirements traceability matrix is a project document that links the project requirements to their origin and tracks them throughout the project life cycle3. It helps to ensure that each requirement adds business value and is aligned with the project objectives. It is useful to review the requirements traceability matrix during the project planning and monitoring, but it is not the main focus when the deliverables are not meeting the quality standards.

Option D is not the best answer because reviewing the work breakdown structure (WBS) is not the next thing to do. The WBS is a project document that decomposes the project scope into smaller, more manageable components called work packages4. It defines and organizes the project work and deliverables. It is helpful to review the WBS during the project planning and execution, but it is not the most relevant document when the deliverables are not functioning as expected. References: 1: PMBOK Guide, 7th Edition, Section 8.1.3.1, Quality Management Plan 2: PMBOK Guide, 7th Edition, Section 13.1.3.1, Stakeholder Engagement Plan 3: PMBOK Guide, 7th Edition, Section 5.2.3.1, Requirements Traceability Matrix 4: PMBOK Guide, 7th Edition, Section 5.4.3.1, Work Breakdown Structure