Final answer:
Nathan Radley claims that the tree is sick as the reason for filling a knot-hole with cement, hindering communication between Boo Radley and the Finch children in 'To Kill a Mockingbird'.
Step-by-step explanation:
The character Nathan Radley gives the reason for plugging the hole in the tree with cement by claiming that the tree is sick. In 'To Kill a Mockingbird' by Harper Lee, Radley's action is interpreted as a means to stop any further communication between his brother, Boo Radley, and the children, Scout and Jem, who have been finding gifts in the knot-hole of the tree. Despite suspicions that the tree is not actually sick, the cement plug symbolically separates Boo from the external world and the children's innocent curiosity.