Topic: Conditional Sentence,

Best Answer Explained in Below-:

The correct choice, C: "If I had known this book was important, I would have read it," is an example of a third conditional sentence, which is used to express hypothetical situations in the past that did not happen.

In the original statement, the speaker expresses regret or a missed opportunity: they did not know that the book was essential for the exam and, consequently, did not read it. The third conditional structure consists of two parts: a past perfect form in the 'if' clause (in this case, "had known") and a conditional perfect form in the main clause ("would have read").

This construction implies that if the speaker had been aware of the book's importance in the past, they would have taken the action of reading it. The options A and B are incorrect because they either misuse verb forms or imply a conditional situation that doesn't reflect the past regret correctly. Hence, option C accurately captures the speaker's sentiment, reflecting their understanding of a past condition that could have led to a different outcome, which makes it the best choice.