Select the option that represents the letters that, when sequentially placed from left to right in the blanks, will complete the letter series.

_ s t _ _ l b z _ t m m _ _ z s _ _ m l b

z m m s l b t m

Given: _s t_ _ l b z_ t m m _ _ z s _ _ m l b

So;

1) z m l  s l b m t --> z s t m l l b z s t m m l b z s m t m l b 

2) z m m s l b t m --> z s t m m l b z s t m m l b z s t m m l b

3) z m m s b l t m --> z s t m m l b z s t m m b l z s t m m l b

4) z m m s l b m t --> z s t m m l b z s t m m l b z s m t m l b

So, In option 2 "z s t m m I b" is repeating itself.

Thus, 'z m m s I b t m' is the correct answer.