'R' will be the first part of the given sequence as the use of the transition word 'then' shows what happened after seventeen years. 'P' will follow next as the use of the word 'there' is used in the context of 'slums of Calcutta' given in 'R'. 'S' will follow next as it shows the effect of reaching out to people living there, that is, she had found her second vocation, her real calling. 'Q' will come next as it states that she was so fascinated with her new calling that she asked the permission to work there. Hence, RPSQ is the correct sequence. Thus, option (b) is correct.