In the following question, which one set of letters when sequentially placed at the gaps in the given letter series shall complete it?
_sr_tr_srs_r_srst_
Option 1 - tsrttrssrssrrsrstr
Option 2 - tsrstrrsrstrssrstr
Option 3 - ssrttrrsrstrrsrsts
Option 4 - tsrstrtsrstrtsrstr
Here, tsrstr follows a pattern.
Therefore, tstttr is the answer.