When a risk that has been previously identified occurs, the project manager should consult the risk register to determine the planned responses for this risk event. The risk register contains the details of risk responses that are planned for the identified risks, including contingency plans and fallback plans. Implementing the planned risk response is crucial to manage the risk effectively and keep the project on track. This approach is aligned with the PMI's guidelines on risk management, which emphasize the importance of preparing for and responding to risks proactively. References: (Project Management Professional (PMP) Reference Materials source and documents; Professional in Business Analysis Reference Materials source and documents)

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.

The Stakeholder Register is a critical document in project management that lists all stakeholders involved in a project, including their roles, requirements, expectations, and influence on the project. The fact that the leader of a user party was not included in the requirements collection communications suggests that the Stakeholder Register was either missed or outdated. This document should have been used to identify all parties with a vested interest in the project, ensuring their needs and expectations are considered during the requirements collection phase. An up-to-date Stakeholder Register is essential for effective communication and project success12.
References:
"What is a Stakeholder Register?" - Project Management Academy1.
"What is a Stakeholder Register in Project Management?" - The Knowledge Academy2.

In an agile project, when there is a disagreement about a major new requirement among key stakeholders, the project manager should facilitate a meeting with the stakeholders and product owner to clarify the new need.
This approach ensures that all parties have a clear understanding of the requirement and can work towards a common goal. It also helps to prevent potential issues that could arise from misunderstandings or miscommunications. References: PMBOKGuide - Sixth Edition, Section 13.2.2.1: Plan Stakeholder Engagement