PS will make a pair as it is making a conditional sentencetogether. Q will come next as the conjunction ‘and’ is making a link with ‘S’, that is, ‘knocked him down and not waited’. ‘R’ will be the last part of the given sequence as ‘why he had written’ is in relation to ‘ask him’ mentioned in ‘Q’. Hence, PSQR is the correct sequence. Thus, option (a) is correct.