He tells them the tree is dying.
Nathan Radley said that when a tree is dying you fill is with cement to save it.
Nathan Radley tells Jem that he filled in the knothole with cement because the tree was dying and he wanted to stop it from spreading. In reality, he likely did it to prevent Boo Radley from continuing to communicate with the children.
Jem is devastated when he discovers the knothole filled with cement. He understands that Nathan Radley had destroyed the only communication Boo had with other people.
Mr. Radley filled the knothole up with cement.
They can not because the knothole is filled with cement by Mr. Radley
That the knothole has been filled with cement. That's what was on Spark Notes.
Nathan Radley knows Boo has been leaving gifts there for the children so fills the hole in with cement. When the children enquire as to why he's doing this, he lies and says that the tree was dying and this would save it.
Mr. Radley puts cement in the knothole to prevent any further communication or interaction between Boo Radley and the children. This action is driven by his desire to isolate Boo Radley from the outside world.
Jem and Scout discovered that someone had filled the knot hole with cement, sealing off the opening where gifts had previously been left for them by Boo Radley. This action by Nathan Radley signified the end of their secret communication with Boo.
Nathan Radley, Boo Radley's brother filled the knothole with cement. When Jem and Scout asked him why he was doing this he told them that his tree was dying and filling the knothole with cement would keep it from dying. Scout and Jem went and asked Atticus if he thought the tree looked like it was dying and he said it looked perfectly healthy.
Nathan Radley filled it with cement
Nathan Radley tells Jem and Scout that he filled the knot-hole with cement because the tree was dying and he didn't want it to fall on someone. However, it is later revealed that he filled it to prevent gifts being left for the children by Boo Radley.