The answer is D) Mr. Nathan cemented the hole closed. In the novel "To Kill a Mockingbird" by Harper Lee, the tree with the knothole in front of the Radley house is a significant symbol that represents mystery and surprises for Jem and Scout.
However, after Mr. Nathan Radley cements the hole closed, Jem and Scout can no longer leave or receive anything from the tree, and the surprises and mysteries that it represents are lost.