Arrange the parts to form a meaningful sentence: P: a decade sooner than 2050 / Q: that the population will peak / R: the second scenario suggested / S: at 8.5 billion by 2040

The complete sentence reads: 'The second scenario suggested (R) that the population will peak (Q) at 8.5 billion by 2040 (S) a decade sooner than 2050 (P).' R introduces the subject and main verb, Q provides what was suggested, S gives the specific peak details, and P adds the comparison. Hence, the correct sequence is RQSP.