None of these answers are good.
Generally, this question seems to be about the Scrum Team's outputs rather than the outcomes enabled by the work completed in the Sprint. The Sprint's purpose is not to complete Product Backlog Items or tasks but to achieve the Sprint Goal. To ensure visibility and transparency, there should be a connection between the Sprint Goal, some of the selected Product Backlog Items, and how the Product Backlog Items are decomposed during Sprint Planning. There's no mention of the Sprint Goal in any of the answers.
Looking into the answers, they are all wrong.
A is incorrect because the implication is that the Definition of Done will be relaxed to allow the work to be considered Done. Relaxing the Definition of Done is the same as decreased quality, and "quality does not decrease" during a Sprint. The best time to review the Definition of Done is at the Sprint Retrospective, so the updated Definition of Done can guide the next Sprint Planning. I'd also hold that quality should not decrease between Sprints, either, so changes to the Definition of Done should not lead to a decrease in the quality of the work performed.
B is wrong since working extra time goes against working at a sustainable pace. Not only is a sustainable pace a part of Scrum, where it promotes focus and consistency, but it's a principle of Agile Software Development.
C is the closest to being right, but it is still wrong. Incomplete Product Backlog Items do return to the Product Backlog, but this happens at the end of a Sprint. The nuances of how this is done depend on the tools being used, but knowing what work was complete, started but incomplete, or not started is useful to facilitate discussions at the Sprint Review and Sprint Retrospective. The team should not remove the Product Backlog Items from the Sprint Backlog until the end of the Sprint, at which point the Product Owner can determine if they are still relevant and order them with the other Product Backlog Items for a future Sprint.
D is incorrect because C is partially right. Work does not roll over automatically between Sprints. Incomplete work must be brought into the next Sprint at Sprint Planning, even if it is started or partially complete. If the work is not valuable, it could end up lower in the Product Backlog or even removed entirely. The Product Owner needs to work with the Developers to weigh the work's state, the cost of incomplete work, and the value of doing the work to decide how it is ordered.
Given the poor quality of the question and answers, I'd also be suspect of the quiz and any training or certifications offered by this organization. It doesn't seem to align well with the Scrum Guide. Looking through the link provided, there are several questions that I disagree with, based on the last two revisions to the Scrum Guide. I'm not sure the answers were ever correct according to the Scrum Guide.