Correct Answer: stasursta

Solution : Given:
s_ _ u _ ar _ st _ aa _ s _ _ ua _ r

To fill the series, we have to divide the series – s_ _ u _ ar / _st _ aa _ / s _ _ ua _ r
Let's check the given options –
First option: stausrsta; s s t u a ar / u st s aa r / s s t ua a r (No repeated pattern found.)
Second option: stasursta; s s t u a ar / s st u aa r / s s t ua a r (sstuaar is repeated in the series.)
Third option: starussta; s s t u a ar / r st u aa s / s s t ua a r (No repeated pattern found.)
Fourth option: stasusrta; s s t u a ar / s st u aa s / s r t ua a r (No repeated pattern found.)

So, the sequence is sstuaarsstuaarsstuaar. Hence, the second option is correct.