What should the agile coach do?

According to the PMI Agile Practice Guide (Section 7.1 Communication) and PMBOK® Guide (6th Edition, Section 13.3 Manage Stakeholder Engagement), transparency with stakeholders is fundamental in agile environments. Delays or deviations from expected outcomes must be promptly communicated to affected stakeholders to adjust expectations and plans.

Mike Griffiths in the PMI-ACP Exam Prep Book (Chapter 4: Stakeholder Engagement) emphasizes that one of the key values in agile is responding to change over following a plan, and that includes openly discussing risks and delays with impacted parties.

Thus, Option D is correct: the agile coach should proactively engage marketing stakeholders to inform them about the delay and support revised planning.