You must leave a note
P : otherwise she will be terribly worried
Q : for your mother
R : at the usual hour
S : if you are not back home

The proper sequence should be:

A. QPRS

B. QPSR

C. SRPQ

D. SRQP

Answer: Option B

Solution(By Examveda Team)

You must leave a note for your mother otherwise she will be terribly worried if you are not back home at the usual hour.