Correct Answer: QQRPR

Solution : Given:
P_P_RPQ_S_Q_ST

Divide the series – P_ / P_R / PQ_S / _Q_ST
Let's check each option –
First option: QRQPR; P Q / P R R / PQ Q S / P Q R ST (No repeated pattern found.)
Second option: QQRPR; P Q / P Q R / PQ R S / P Q R ST ( PQ is repeated in every part of this series and one letter is added at every step.)
Third option: PQPRR; P P / P Q R / PQ P S / R Q R ST (No repeated pattern found.)
Fourth option: PRQRQ; P P / P R R / PQ Q S / R Q Q ST (No repeated pattern found.)

So, the series becomes→PQPQRPQRSPQRST. Hence, the second option is correct.