At the start of iteration 3 of 6, planning shows some requirements are not defined clearly enough to begin. What should the project manager do?

The correct action is to meet with the product owner to review the project backlog. In Agile, the product owner is responsible for defining and prioritizing backlog items, including requirements. If requirements are unclear, the product owner is the primary person to clarify them, refine their details, or re-prioritize them for a future iteration if they cannot be clarified immediately. This ensures the team is working on well-understood and valuable features. Reviewing and replacing unclear requirements is part of the solution, but the product owner is the one who ultimately owns this process, not the project manager directly. Logging the issue and escalating to the sponsor is premature; the product owner is the first point of contact for backlog issues. Inviting key users to a daily stand-up is inappropriate for detailed requirement clarification; daily stand-ups are for team synchronization, and detailed discussions should happen separately, often involving the product owner.