UPSC CDS English Ordering of Words in a Sentence Part 1 Questions

‘At a time when’ is a logical phrase which forms the pair of RQ. Now Q is followed by S as S cannot logically follow P. Considering the chronology of RQS, P cannot be the first part of the sentence hence it should be the last part. Among the given options, only (b) contains this chronological order. Thus, it is clear that the correct option is (b)