According to the Scrum Guide, the Developers do the work to make sure Product Backlog items conform to the Definition of Done, as they are responsible for creating a "Done" Increment that meets the Definition of Done. The other options are not valid, as they imply that the work is done by someone else outside the Development Team (such as Quality Assurance Team) or by another role on the Scrum Team (such as Product Owner or Scrum Master).