PO Stand-in?

This is a typical PSPO test question beleive it or not. Answer is that Developers must do their best working for Sprint goal on selected PBI items, until PO will not be available again.

If there is Scrum planning before PO 's return they should select PBI for Sorint backlog in his or her absence.

If PO will be permanently unavailable, there should be nee PO in a Scrum team.

Remember that the Scrum Guide does not define job titles or provide job descriptions.  It provides roles.  People with any job title can fulfill the roles as long as they follow the guidelines from the Scrum Guide.  Ideally, each of the roles would be fulfilled by unique individuals.  But in some situations like the one you described, it is possible for someone to step in and carry on the responsibilities of the role.  

I have been in similar situations.  What my teams did were to discuss each situation and decide on how they felt it was best to handle it.  We would use internal stakeholders to help, we would use an individual on the team to act as "PO for this Sprint" because they understood the work better than anyone else.  There are many ways to deal with the situation but I have found that each situation has unique qualities that make it easier to handle each one individually.  For example, the time the individual that is normally fulfilling the PO role will be away is a big factor.  So is the work that is at the top of the Product Backlog.  

@Nicholas is correct in his answer and that this is a typical PSPO question.  I feel it is asked in order to see if you fully understand that no single individual is required for the Scrum Team to continue to progress when someone is missing.  But as @Nicholas' last statement states, there does need to be a someone that fulfills the role of Product Owner and that should be an individual that the organization respects to make the right decisions for the Product direction.

If a PO is on holidays or on sick leave for maybe 3 or 4 weeks - who will take over the PO-role for this time?

Nobody. The Product Owner remains accountable. Product Ownership isn't a mere role which is taken up or put down. Rather, it describes an accountability and a commitment to the optimization of product value.

This should not create a problem, but it might very well reveal one. If the team can't self-manage effectively until the PO comes back, find out why. Appointing a stand-in might just cover up a deeper problem.

Thank you all so much! Your answers really help me.