'R' will be the first part of the given sequence as 'that is why children' is used in order to give the reason for the statement given in S1. 'Q' will follow next as the pronoun 'it' is used in relation to 'new language' mentioned in 'R'. SP will make pair together as both the sentences are talking about knowing the language and the ability to use it. Hence, RQSP is the correct sequence. Thus, option (d) is correct.