It is the prerogative of the Product Owner to define the business objective of an upcoming Sprint by identifying and ranking the most valuable user stories in the Product Backlog, and it is the duty of the Scrum Master to support the Product Owner in this. Pursuant, a suitable way for a Scrum Master to support a Scrum Team’s strive to work on the most valuable Product Backlog items is:
To ensure that the Scrum Team is involved in the product discovery process at an early stage;
To ensure that the Product Backlog refinement process is well practiced by both the Development Team members and the Product Owner; and
To ensure that all user stories are created in a collaborative effort between the Product Owner and the Development Team members (the goal being a shared understanding of the user stories and thus joint ownership).
Your candidate should note that although the Product Owner practically outlines the scope of the Sprint, it is the prerogative of the Development Team to address technical debt and bugs during the same Sprint. A Development Team should be able to allocate up to 25% of their available capacity for this.