This question belongs to the People domain because it focuses on accountability, role clarity, and team performance. A responsibility assignment matrix (RAM) is used to clarify who is responsible and accountable for specific work. Since the project already has a well-established RAM, the project manager should first use that artifact to identify the task owner and then discuss why the task remains open. That is the most direct and professionally appropriate response, and it aligns with PMI's emphasis on supporting team performance, clarifying accountability, and addressing issues through the proper channels.
Option A ignores the established accountability structure and may create confusion by bypassing the assigned owner. Option B is inappropriate because the stakeholder has raised a legitimate concern about incomplete work. Option D is unnecessary because accountability should already be defined through the RAM. PMP- style thinking favors using existing governance and responsibility structures before creating new ones.
Therefore, the best action is to identify the owner and address the situation directly with that person, ensuring transparency, corrective action, and continued accountability within the team.