During sprint planning the team can deliver only about 95% of the selected items. The product owner asks for advice on prioritizing the backlog. What should drive the prioritization?

Business value should be the primary driver for prioritizing the backlog. The product owner's role is to maximize the value of the product resulting from the work of the development team. By prioritizing items that deliver the highest business value, the team ensures that the most important features are developed first, even if not all selected items can be completed. Technical complexity, while a factor in estimation and planning, should not solely drive prioritization as a highly complex but low-value item might be deferred. Risk to delivery is important for planning but doesn't define the inherent worth of an item. Estimation accuracy is a measure of the team's ability to size work, not a criterion for its importance.