According to the Scrum Guide1, the Product Backlog is an ordered list of everything that is known to be needed in the product. It is the single source of requirements for any changes to be made to the product. The Product Owner is accountable for maximizing value resulting from work by Developers. The Product Owner is also accountable for effective Product Backlog management, which includes:
Developing and explicitly communicating the Product Goal;
Creating and clearly communicating Product Backlog items;
Ordering Product Backlog items; and
Ensuring that the Product Backlog is transparent, visible and understood.
Therefore, the Product Owner is responsible for clearly expressing Product Backlog items.

According to the Scrum Guide1, the Scrum Master is accountable for establishing Scrum as defined in the Scrum Guide. They do this by helping everyone understand Scrum theory and practice, both within the Scrum Team and the organization. The Scrum Master serves the Developers in several ways, including:
Coaching them in self-management and cross-functionality;
Helping them to create high-value products;
Removing impediments to their progress;
Facilitating Scrum events as requested or needed; and,
Coaching them in organizational environments in which Scrum is not yet fully adopted and understood.
The Scrum Master serves the Product Owner in several ways, including:
Helping find techniques for effective Product Goal definition and Product Backlog management; Helping the Scrum Team understand the need for clear and concise Product Backlog items; Helping establish empirical product planning for a complex environment; and, Facilitating stakeholder collaboration as requested or needed.
The Scrum Master serves the organization in several ways, including:
Leading, training, and coaching the organization in its Scrum adoption; Planning and advising Scrum implementations within the organization; Helping employees and stakeholders understand and enact an empirical approach for complex work; and, Removing barriers between stakeholders and Scrum Teams.
Therefore, two things that are true about the Scrum Master role are:
The Scrum Master teaches the Development Team to keep the Scrum meetings to their time-box, as they help everyone understand Scrum theory and practice.
The Scrum Master helps those outside the team interact with the Scrum Team, as they serve both the Product Owner and the organization.
The other options are not true about the Scrum Master role, as they imply that they have authority or responsibility over things that are not within their accountability.

A Scrum Team is a cohesive unit of professionals that consists of one Product Owner, one Scrum Master, and Developers, as stated in the Scrum Guide: "The Scrum Team consists of one Scrum Master, one Product Owner, and Developers. Within a Scrum Team, there are no sub-teams or hierarchies. It is a cohesive unit of professionals focused on one objective at a time, the Product Goal."

According to the Scrum Guide, a Sprint concludes when the Sprint Retrospective is complete. This is the last event of the Sprint cycle, where the Scrum Team inspects itself and creates a plan for improvements. The other options are not valid indicators of the conclusion of a Sprint, as they are either irrelevant (such as the Product Owner deciding enough has been delivered) or incomplete (such as all tasks or Product Backlog items being done).