Correct Answer: RTSQ

Solution : Given:
QST _ QS _ RQ _ TR _STR

To fill the series we have to divide the series – QST _ / QS _ R / Q _ TR / _STR
Let's check each option –
First option: SQTR; QST S / QS Q R / Q T TR / R STR (No repeated pattern has been found)
Second option: RTSQ; QST R / QS T R / Q S TR / Q STR (QSTR is repeated in the series)
Third option: TRQS; QST T / QS R R / Q Q TR / S STR (No repeated pattern has been found)
Fourth option: TSRQ; QST T / QS S R / Q R TR / Q STR (No repeated pattern has been found)

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