Question: _ stt _ tt _ tts _
[A].tsts 
ttst 
  
  
sstt 
tsst 
[A].
[B].
[C].
[D].
Answer: Option D
Explanation:
The series is tst/tst/tst/tst. Thus, the pattern ‘tst’ is repeated.
[B].
[C].
[D].
Answer: Option D
Explanation:
The series is tst/tst/tst/tst. Thus, the pattern ‘tst’ is repeated.