Correct Answer: tstttr

Solution : Given:
_sr_tr_srs_r_srst_

To fill the series we have to divide the series – _sr_tr /_srs_r /_srst_
Let's check each option –
First option: ttssrr; t sr t tr / s srs s r / r srst r (No repeated pattern has been found.)
Second option: tsrtsr; t sr s tr / r srs t r / s srst r (No repeated pattern has been found.)
Third option: strtrs; s sr t tr / r srs t r / r srst s (No repeated pattern has been found.)
Fourth option: tstttr; t sr s tr / t srs t r / t srst r (tsrstr is repeated in the series.)

So, the series becomes→tsrstrtsrstrtsrstr. Hence, the fourth option is correct.