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

t _ r a s _ t _ s _ a _ _ _ t _ r _ s r t

s r t r s r t s a

By using options one by one .

 t _ r a s _ t _ s _ a _ _ _ t _ r _ s r t

Option 1:   t s r a s r t /t s r a a r  t/ t s r a s r t

Option 2:   t s r a s r t /a s r a s r  t/ t s r a s r t

Option 3:   t s r a s r t /t s r a s r  t/ t s r t s r t

Option4:   t s r a s r t /t s r a s r  t/ t s r a s r t

Series pattern: t s r a s r t /t s r a s r  t/ t s r a s r t.

Therefore, only s r t r s r t s a follows the pattern.

Hence, the correct answer is s r t r s r t s a.