PO concerned about all sprint backlog items not being completed
The PO on a project shares his concern with the Scrum-master that it doesn't feel like dev team would be able to fulfill their commitment to deliver the PBIs selected during Sprint Planning. As a scrum master which action should I take?
Option 1: Tell the PO that no one tells the dev team on how to turn the sprint backlog into potentially releasable functionality. It is up to the Dev team to meet their commitments.
Option 2: Coach the PO on complex software development and tell her that the work (sprint backlog) emerges during sprint and so all the PBIs that were forecasted for the sprint cannot be completed everytime.
Can I suggest a 3rd option? This isn't just about coaching the PO, it's about coaching the dev team to have the skills to communicate to the PO (and others) why their selected Sprint Backlog is appropriate and sensible.
Although you're right that ultimately only the dev team can decide what's in the sprint backlog, that doesn't mean they shouldn't take input into those decisions from others, particularly the PO.
Discuss with the PO what their concerns are and encourage them to bring them to the dev team. Either the dev team should be able to set the PO's mind at rest, or they may realise they've not taken a factor into account and are actually in danger of not meeting the sprint goal. E.G. the PO may be aware a PBI is more complex than the dev team seem to be treating it, or have noticed that they've committed to significantly more items than they've previously successfully delivered.
You may also have a problem with artifact transparency. It may help reassure the PO if they can see an accurate depiction of the current state of the sprint backlog, so they can see if the team is on track to meet the sprint goal.