Explanation
The PO's best course of action is to communicate the status of the Iteration to all stakeholders, including the other teams on the Agile Release Train (ART), the Release Train Engineer (RTE), the System Architect/Engineer, the Product Management, and the Business Owners. This will help to align expectations, manage dependencies, and mitigate risks. The PO should also collaborate with the team and the stakeholders to prioritize the remaining work and identify the most valuable Stories to deliver by the end of the Iteration.
The PO should not defer acceptance testing to the next Iteration, as this would violate the Definition of Done and compromise the quality of the system increment. The PO should not have an emergency Iteration Planning meeting, as this would disrupt the cadence and synchronization of the ART and waste time and resources. The PO should not stop the current Iteration and plan a new Iteration with the new knowledge, as this would also disrupt the cadence and synchronization of the ART and create confusion and uncertainty.
References: Team Backlog - Scaled Agile Framework, Iteration Planning - Scaled Agile Framework, Iteration Execution - Scaled Agile Framework