According to the Project Management Professional (PMP) standards, when a milestone is missed, it’s important to maintain transparency and communicate effectively with all stakeholders. This includes sending formal communication about the missed milestone. While this may reflect poorly on the team in the short term, it upholds the principles of accountability and integrity, which are crucial for long-term success and trust-building in any project.

Delaying communication (Option B) or foregoing it (Option D) can lead to a lack of trust and can create a precedent for non-transparency. Asking for a meeting with the sponsor to decide whether to send the formal communication (Option C) could be seen as an attempt to avoid responsibility. Therefore, the best course of action is to send a formal communication regarding the missed milestone (Option A).

Project Management Professional (PMP) Certification Handbook available at PMI’s official website. Specifically, refer to the section on ‘Professional Responsibility’ which discusses the importance of transparency and communication in project management.