Correct option is A
The correct substitution of the highlighted segment is “need a chair to sit on.”
Error identification: The phrase "to sit" is incomplete because it lacks the preposition "on" to indicate the surface or object on which one would sit.
Grammatical Rule used: The correct phrase "to sit on" clearly indicates the object (a chair) on which the action (sitting) will occur.
Corrected sentence: I am very tired and need a chair to sit on.