‘Q’ will be the first part of the given sequence as ‘small pox’ forms the subject. ‘P’ will come next because it states that it was a dreaded disease up to the end of eighteenth century and moreover the pronoun ‘it’ is used in the context of ‘small pox’ referred in ‘Q’. ‘R’ will follow next as ‘not only..but also’ is a correlative conjunction and hence, should be in a single given sentence. ‘S’ will come next as ‘permanently disfigured’ is used in relation to ‘those who recovered’ in R. Hence, QPRS is the correct sequence. Thus, option (d) is correct.