The work to make sure Product Backlog items conform to the Definition of Done is done by the Developers. This is because:

The Developers are accountable for creating a “Done” Increment in every Sprint. They must ensure that every Product Backlog item they work on meets the Definition of Done before it is considered complete.

The Definition of Done is a formal description of the state of the Increment when it meets the quality measures required for the product. It is a shared understanding among the Scrum Team and the stakeholders of what “Done” means for any Product Backlog item that is selected for a Sprint.

The Developers are self-managing professionals who organize and manage their own work. They decide how to best accomplish their work, rather than being directed by others outside the Scrum Team.

Other options, such as the Product Owner, the Quality Assurance Team, the Scrum Team, or the Scrum Master, are not responsible for making sure Product Backlog items conform to the Definition of Done. They may have different roles and accountabilities in Scrum, but they do not do the actual work of creating a “Done” Increment.

References:

[Scrum Guide], page 7, section “Developers”

[Scrum Guide], page 10, section “Definition of Done”

[Scrum Guide], page 7, section “The Scrum Team”