This question fits the People domain because it involves stakeholder expectation management and handling late-emerging requirements. PMI guidance notes that stakeholder pressure often generates change and increases project complexity, so project managers must actively manage expectations and use structured change handling. PMI also states that all requirement changes should be handled through an agreed change control process. Since the prototype was already approved and final development has begun, the most practical PMP-aligned response is to acknowledge the concern and position the new requirement for a future deliverable cycle rather than disrupt current approved work.
Option A is useful as a lesson for future engagement, but it does not resolve the immediate request. Option B is too dismissive and damages stakeholder relationships. Option C is confrontational and not consistent with PMI's collaborative approach to stakeholder management. Option D best balances project stability with stakeholder value by showing the requirement is heard, while protecting the approved scope and ongoing delivery. This reflects disciplined scope management and constructive stakeholder communication rather than rejecting the stakeholder or reopening completed approvals without process.