Little
P : that he had been let down
Q : stood by all these years
R : did he realise
S : by a colleague whom he had

The Proper sequence should be:

A. QSPR

B. QSRP

C. RPSQ

D. RSQP

Answer: Option C

Solution(By Examveda Team)

Little did he realise that he had been let down by a colleague whom he had stood by all these years.