The correct answer is 'Had I realised'. Key Points Here, in the given question the most appropriate option is 'Had I realised'. The given sentence is an example of a third conditional. Its structure as follows: ​Structure of 'Third Conditional': Had+ Subject +V3, S + would + have +V3. Example: Had I seen you, I would have stopped my car. Therefore, as per the points mentioned above, we find that the correct answer is Option 3. ​ Correct Sentence: Had I realised the nature of the job earlier, I would not have accepted it. Additional Information Other Two Important Structures of Conditional Sentences: Structure of 'First Conditional': If +Present Indefinite, Simple Future​. Example: If he works hard, he will surely get the job. Structure of 'Second Conditional': If+Subject+V2, Subject+would+V1. Example: If I won the lottery, I would buy a big house.