Concept:The letter series is completed by identifying a repeating block.Explanation:Write the series with blanks: NO_PQN_PP_NOP_Q.Using option B, fill the blanks with POQP.The completed series becomes NOPPQNOPPQNOPPQ.This is clearly three repetitions of the block NOPPQ.So the required set is POQP.Answer:Option B: POQP