'Q' will be the first part of the given sequence as it states the 'immense boon' referred in the opening part. 'S' will follow next as it talks about the improvement in the death rate of children due to the availability of vaccination in nineteenth century. 'P' will come next because the pronoun 'it' is used for the 'vaccination' in 'S'. 'R' will come next as it shows the effect of vaccination in the countries such as England and Wales. Moreover, it makes a link with S6 as it says that as a result of continuous vaccination, death rate in 1948 (10.8) was the lowest ever recorded up to that date. Hence, QSPR is the correct sequence. Thus, option (b) is correct