The correct answer is: E. No error
The sentence is grammatically correct. The conjunction “but” is used to join two independent clauses, which are clauses that could stand on their own as sentences. In this case, the two independent clauses are “His father told me that though his son had worked very hard” and “he had failed to make any mark in the examination.” The conjunction “but” is used to contrast the two clauses, indicating that the son’s hard work did not result in success in the examination.
The other options are incorrect because they suggest that there is a grammatical error in the sentence. Option A is incorrect because it omits the word “that” after the verb “told.” Option B is incorrect because it omits the comma after the word “though.” Option C is incorrect because it omits the comma after the word “hard.” Option D is incorrect because it omits the word “the” before the word “examination.”