S1 : The Subsidiary Alliance system was extremely advantageous to the British.
S6 : They controlled the defence and the foreign relations of the protected ally.
- P : They could now maintain a large army at the cost of Indian states.
- Q : if many war occurred in the territories
- R : either of the British ally or of the Britishers
- S : This enabled them a to fight wars far away from their own territories
This proper sequence should be
- P Q R S
- P S Q R
- Q R P S
- S R P Q
The paragraph is about Subsidiary Alliance system. "They" in sentence P refers to the British. "This" in the sentence S refers to maintaining large armies. So, S follows P.
The correct option is B.