Scrum roles: Product Owner, Scrum Master and Developers
A Scrum Team is one Product Owner, one Scrum Master and Developers, with no sub-teams or hierarchies. The Product Owner maximizes product value and manages the Product Backlog, the Scrum Master makes the team effective and establishes Scrum, and the Developers plan and build each Increment.
The three accountabilities
Developers are accountable for creating the Sprint Backlog, instilling quality by adhering to the Definition of Done, adapting their plan each day toward the Sprint Goal, and holding each other accountable as professionals.
The Product Owner is accountable for maximizing the value of the product and for Product Backlog management: developing and communicating the Product Goal, creating and communicating Product Backlog items, ordering them, and keeping the backlog transparent. The Product Owner is one person, not a committee.
The Scrum Master is accountable for establishing Scrum as the Scrum Guide defines it and for the Scrum Team's effectiveness.
Delegation does not move accountability
The Product Owner may do the backlog work or delegate it to others, but remains accountable. If the Developers wrote an unclear backlog item at the Product Owner's request, the Product Owner still answers for it.
Self-managing and cross-functional
Scrum Teams are cross-functional, meaning the members have all the skills needed to create value each Sprint, and self-managing, meaning they decide internally who does what, when and how. The team is typically 10 or fewer people.
That is why naming a lead Developer to hand out work each day conflicts with Scrum, and why a Product Owner who tells the Developers exactly how to build and split each item at Sprint Planning is overstepping. The Scrum Guide says how selected items become an Increment is at the sole discretion of the Developers.
Where the exam catches people
Believing a Scrum Team has a project manager or team lead role: the Scrum Guide defines only Product Owner, Scrum Master and Developers.
Believing whoever writes a backlog item is accountable for it: the Product Owner stays accountable after delegating.
Believing cross-functional means members come from many departments: it means the team has all the skills needed.
Believing the Product Owner decides how work is built: the Developers alone plan how to turn items into an Increment.