It is
P : the king and his four sons
Q : believed that
R : when they were riding in a royal procession
S : were killed

The Proper sequence should be:

A. PQSR

B. PSQR

C. QPRS

D. QPSR

Answer: Option D

Solution(By Examveda Team)

It is believed that the king and his four sons were killed when they were riding in a royal procession.