According to the Project Management Body of Knowledge (PMBOK) guide, the project manager should ensure that the project meets the agreed acceptance criteria by performing tests and routinely tracking results as per the quality management plan. This approach allows the project manager to monitor the project's progress and make necessary adjustments to ensure that the project meets the acceptance criteria. References:
PMBOK Guide, 6th Edition, Section 8: Quality Management

In a project, when a task on the critical path is at risk of delaying the schedule, it's crucial for the project manager to identify any impediments that might be causing the delay. In Agile methodologies, such as Scrum, this is often done during daily standup meetings. If a team member has been providing the same update for several days, it may indicate that they are facing a blocker. The project manager should ask the team member if there is an impediment and help to remove the blockers. This approach promotes problem-solving and helps keep the project on track. References: PMBOK Guide, 6th Edition, Section 8.3 - Control Schedule

According to the PMP Exam Content Outline, one of the tasks under the domain of Stakeholder Management is to manage stakeholder engagement, which includes communicating and interfacing with stakeholders to meet their needs and expectations, address issues, and foster appropriate stakeholder involvement.
Stakeholder engagement is the process of involving stakeholders in the project decisions and activities, and ensuring that their interests and concerns are considered and addressed. Stakeholder engagement is crucial for project success, as it can increase stakeholder satisfaction, commitment, support, and trust, as well as reduce conflicts, risks, and resistance to change12.
One of the tools and techniques for managing stakeholder engagement is the stakeholder engagement plan, which is a component of the project management plan. It describes the strategies and actions that will be used to promote productive involvement of stakeholders in decision making and project execution3. The stakeholder engagement plan should include the following elements:
* Stakeholder identification and analysis: This involves identifying the key stakeholders of the project, their roles, interests, expectations, influence, and impact on the project, and their current and desired levels of engagement.
* Stakeholder engagement assessment matrix: This is a table that shows the changes in the level of engagement of each stakeholder or stakeholder group over time, and the gaps between the current and desired levels of engagement.
* Stakeholder engagement strategies: These are the approaches and methods that will be used to increase the level of engagement of stakeholders, such as communication, negotiation, collaboration, consultation, education, etc.
* Stakeholder communication plan: This is a document that specifies the information needs and preferences of each stakeholder or stakeholder group, the frequency and mode of communication, the communication channels and methods, the communication roles and responsibilities, and the communication feedback and evaluation mechanisms.
* Stakeholder engagement action plan: This is a list of specific actions and activities that will be performed to implement the stakeholder engagement strategies and the stakeholder communication plan, such as conducting meetings, workshops, surveys, interviews, presentations, reports, etc.
One of the best practices for stakeholder engagement is to set clear meeting expectations by sharing agendas in advance. This can help the project manager to obtain participation from key stakeholders who do not usually attend project meetings, by:
* Showing respect for the stakeholders' time and availability, and inviting them to attend only the meetings that are relevant and important for them.
* Providing them with the purpose, objectives, topics, and outcomes of the meeting, and how their input and feedback will be valued and used.
* Encouraging them to prepare for the meeting, and to raise any questions, issues, or concerns beforehand or during the meeting.
* Creating a positive and collaborative atmosphere for the meeting, and facilitating the discussion and interaction among the participants.
* Following up with the stakeholders after the meeting, and sharing the meeting minutes, action items, and decisions.
The other options are not correct because they do not address the root cause of the lack of stakeholder participation, which is the low level of engagement. Distributing the communications management plan may not be effective, as the stakeholders may not read it or find it relevant. Rescheduling meetings to align with the stakeholders' availability may not be feasible, as the stakeholders may have conflicting schedules or priorities. Escalating the issue to senior management may not be appropriate, as the stakeholders may perceive it as a threat or a sign of disrespect. Therefore, the best option is to set clear meeting expectations by sharing agendas in advance, as this can increase the stakeholder engagement and participation in the project meetings. References:
* PMP Exam Content Outline
* Stakeholder analysis4
* Stakeholder engagement plan3
* 7 Benefits of Stakeholder Participation1
* Project Stakeholder Management According to the PMBOK2

According to the PMBOK Guide, a resource management plan is a component of the project management plan that describes how project resources are acquired, allocated, monitored, and controlled. It also includes the roles and responsibilities of project team members and other stakeholders, as well as the resource calendars and the process for managing changes to resource assignments. When a key subject matter expert (SME) leaves a multiyear project due to organizational downsizing, the project manager should refer to the resource management plan to determine how to replace the SME, how to transfer the SME's knowledge and expertise to the remaining team members, and how to communicate the change to the relevant stakeholders.
The other options are not the best actions to take in this situation. Updating the stakeholder register is a possible consequence of the change, but not the first thing to do. Consulting the project sponsor may be helpful, but not as effective as following the established plan. Modifying the statement of work (SOW) is not necessary unless the scope of the project is affected by the loss of the SME. References: PMBOK Guide, 6th edition, pages 313-314, 318-319.