Some parts of the sentence have errors and some have none. Find out which part of the sentence has an error. The number of the part is your answer. If there is no error, your answer is (4) i.e. No error.
No sooner she had realized (1)/ her blunder than she began (2)/ to take corrective measures. (3)/ No error (4).
1
1
2
2
3
3
4
4