‘R’ will be the first part of the given sequence as the verb needsto come as per subject-verb agreement, that is, ‘to buy a scooter’. ‘S’ will come next as it contains the object, that is, ‘his son’. ‘Q’ will come next as it begins with the reflexive pronoun, ‘who’ and refers back to the object. ‘P’ will come at the end as it further clarifies the purpose for the action performed by the boy. Hence, RSQP is the correct sequence. Thus, option (d) is correct.