Which one set of letters when sequentially placed at the gap in the given letter series shall complete it. r  s _ p q _ s t p  _  r _ t p q

prqs  trps  trqs  trqp

trqs

Here series is ,   r  s (t)  p q /  (r) s t p  (q)  / r  (s) t p q   sp answer is = trqs