This question tests the correct use of clauses when expressing reasons. The sentence given is: "The reason for (a)/his failure is because (b)/he did not work hard. (c)/No error (d)". Let us analyze the statement and options:
To correct the sentence: "The reason for his failure is that he did not work hard."
Hence, the option containing the error is (b).
Unless you do not listen to his advice, (a) I am not going (b) to help you. (c) No error (d)
The teacher called Ravi (a)and asked him (b) to describe about the incident. (c) No error (d)