According to the PMBOK Guide, an issue log is a project document that records and tracks any issues that arise during the project and how they are resolved. It is part of the project communications management knowledge area and the monitor communications process. A project manager should update the issue log whenever a new issue is identified, analyzed, or resolved. Updating the issue log and defining the mitigation strategy is the immediate action that the project manager should take to improve the situation of a new team member impacting the project performance. This will help to document the issue, communicate it to the relevant stakeholders, and plan how to address it effectively. Communicating the issue to the human resources department (A) may be a subsequent action, but not the immediate one. Providing additional training to the new hire (B) may be a possible mitigation strategy, but not an action to take before updating the issue log.
Increasing team cohesiveness by sharing information and experiences openly may be a good practice for team development, but not a direct response to the issue at hand. References: PMBOK Guide, 6th edition, pages 377-378, 529-530.