According to the PMBOK Guide, an impediment board is a visual tool that tracks the impediments or blockers that prevent the team from completing their work1. The project manager should create an entry in the impediment board and assign it to the person or team responsible for resolving it. This will help the project manager monitor the progress of the impediment resolution and communicate it to the stakeholders. The other options are not the best actions for the project manager to do. Inviting the project team to a swarming session to define a solution is a technique that can be used to address complex or urgent problems, but it is not appropriate for an issue that is outside the project team's control2. Leveraging the network of the former department and asking for help is a possible way to expedite the completion of the task, but it is not a formal or transparent way to manage the impediment. Escalating the issue to the product owner to define the next steps is not the project manager's responsibility, as the product owner is mainly concerned with the product vision and value, not the project execution3. References:
1: PMBOK Guide, 6th edition, page 570
2: PMBOK Guide, 6th edition, page 571
3: PMBOK Guide, 6th edition, page 49