The Sprint will complete in some days when the reaiming tasks require more days.

@Salah, I am not sure where this question comes from or why someone would say A is correct answer, but it is not. You don't change the DoD to handle unfinished work.  There is no mention of the Sprint Goal here and many other flaws. I would suggest that you throw out whatever this test is telling you as it is poorly written and wrong. 

I really dont understand why the correct answer is A.

That's not the issue, the problem is that someone else believes it is right and considers their view to be authoritative. Are you in a position to ask them to present their reasoning here?

That question is so full of misunderstandings that I agree with @Eric.  Stop using that source because they are feeding you misinformation.  

For example, who is to say that the estimate of 2 hours will actually take 2 hours to finish once the work begins.  Once work begins, they will find additional information that could change the actual working time to 30 minutes or 8 hours.  An estimate is only good at the time it is made based upon the information that is available.  So in reality it is possible that none of those Sprint Backlog Items could be completed. 

Also, the Scrum Guide never says anything about tasks.  It refers to Product and Sprint Backlog Items.  So that entire question should be asking about the completion of Product Backlog Items and all estimates should be for them. 

I could go on but I hope you get the picture.  That source is not providing you questions/scenarios based upon the Scrum Guide.  As @Ian said, they are providing you their opinion of how they believe it should be.  If they do not provide a mechanism for you to query them to get their feedback on why an answer would be correct in their opinion, then I wouldn't trust them to provide me a valid knowledge base.

Thank you all for your feedback.

I found this Question /Response document on the internet. I'll just delete it.