In the event of a team member's sudden unavailability, the project manager should first log the situation in the issue log. This is a document where all the issues are recorded and tracked. It's used to document and monitor the status of all identified issues that can impact the project's schedule, cost, or scope. The issue log helps the project manager to analyze the issue, assess its impact, and plan the necessary actions to manage it.
While options A, B, and D might be part of the subsequent actions, the first step should be to log the issue.
Holding an emergency meeting (option A) might be necessary, but it should be done after recognizing and logging the issue. Asking the HR manager for an external replacement (option B) might be a potential solution, but it's not the immediate action. Updating the stakeholders (option D) is also important, but it should be done after the issue has been logged and a plan of action has been determined.
References: Project Management Body of Knowledge (PMBOK) Guide, 6th Edition, Section 11.2: Identify Risks.