Hello! I would say your answer is C, court opinion.
A court opinion is something used in court - it announces the final decision of the judge, as well as restates a lot of the important parts of the court case. This has to do with the law, so it should be very formal.
On the other hand, a letter to your grandmother should be casual as it is a personal matter. A newspaper article is going to be as formal, but it is not regarded with as high importance as something on the legal side. An instruction manual will also be formal, but, same reasoning as a newspaper article.
Hope this helps!