This question belongs to the Process domain because it concerns agile execution controls and completion criteria. PMI defines the definition of done (DoD) as the criteria for accepting a feature or story as finished, and the PMI Lexicon similarly describes it as the checklist of criteria required for a deliverable to be considered ready for customer use. If features repeatedly fail the iteration-level completion standard and are returned to the backlog, the strongest indication is that the team is not consistently building and validating work against the agreed DoD.
Option B is incorrect because extending iterations weakens agile cadence and does not solve the root cause.
Option C can help in some cases, but acceptance criteria are narrower than the broader team agreement captured in the DoD. Option D only delays correction and allows the same problem to continue. PMP-style reasoning favors reinforcing explicit completion criteria and making them visible and consistently applied.
Therefore, the best response is to put more emphasis on the definition of done so backlog items are completed to the required standard before being considered finished.