The stranger considered the student's behaviour to be unbearable. From the list of words lettered A to D, choose the word which is closest in meaning to the underlined word in the sentence.

The word "unbearable" means unable to be endured or tolerated.

Comparing the given options:
A. acceptable - This means satisfactory or able to be agreed to, which is the opposite of unbearable.
B. annoying - This means causing irritation or annoyance. While something unbearable can be annoying, "annoying" is not the closest synonym for the intensity of "unbearable."
C. funny - This means causing amusement or laughter, which is unrelated.
D. intolerable - This means unable to be endured, which is a direct synonym for unbearable.

Therefore, the word closest in meaning to "unbearable" is "intolerable".

The correct answer is D.