Final answer:
Nathan Radley claims he filled the knothole with cement because the tree was sick, an excuse that the children doubt as they suspect his true motive was to prevent Boo Radley from communicating with them.
Step-by-step explanation:
Nathan Radley gives the children the excuse that the knothole was filled with cement because the tree was sick. This explanation is perceived with skepticism by the children who believe that Mr. Radley's actions were not motivated by concern for the tree, but rather to stop the interaction between Boo Radley and the children.