It was to be
P : before their school examination
Q : which was due to start
R : the last expedition
S : in a month's

The Proper sequence should be:

A. SRQP

B. RQSP

C. RPQS

D. SPRQ

Answer: Option C

Solution(By Examveda Team)

It was to be the last expedition before their school examination which was due to start in a month.