According to the PMBOK Guide, a project manager should be able to lead and support the team in achieving the project objectives, as well as manage team conflicts and issues. When a key project team member exhibits a sudden decrease in participation and performance, the project manager should schedule a meeting to discuss recent observations with the team member. This option demonstrates the project manager's ability to communicate effectively, provide feedback, and address the root cause of the problem. The project manager should also show empathy and respect, and explore possible solutions or support that the team member may need. Reallocating tasks for the team member of concern, organizing a team-building session with all of the team members, or consulting the resource allocation matrix are not effective or appropriate actions to take in this situation. These options may ignore, avoid, or worsen the issue, and may not address the individual needs or concerns of the team member. They may also indicate a lack of leadership, interpersonal, or problem-solving skills on the part of the project manager. References: PMBOK Guide, 6th edition, pages
52, 335, 349.