Correct Answer: zmmslbtm

Solution : Given:
_st_ _lbz_tmm_ _zs_ _mlb

To fill the series, we have to divide the series – _st_ _lb / z_tmm_ _ / zs_ _mlb
Let's check each option –
First option: zmlslbmt; z st ml lb / z s tmm lb / zs mt mlb (No repeated pattern has been found.)
Second option: zmmslbtm; z st m m lb / z s tmm l b / zs t m mlb (zstmmlb is repeated in the series.)
Third option: zmmsbltm; z st m m lb / z s tmm b l / zs t m mlb (No repeated pattern has been found.)
Fourth option: zmmslbmt; z st m m lb / z s tmm l b / zs m t mlb (No repeated pattern has been found.)

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