Select the most appropriate option that can substitute the highlighted segment in the given sentence.
Mr. Raghav racked his brain, but couldn’t think where he had left the book.

Correct option is A

The most appropriate option that can substitute the highlighted segment in the given sentence is (a) “thought very hard”.
 The highlighted segment means to think hard or make a great effort to remember something.
 Option (b) "had a series of consecutive thoughts" does not convey the intended meaning of the sentence.
 Option (c) "knew it, but couldn’t quite remember it" implies that the person knows the answer but cannot fully remember it, which is not the same as thinking hard to remember it.
 Option (d) "thought he had heard something before" does not convey the intended meaning of the sentence.