Correct Answer: baab

Solution : Given:
aa_a_b_abaa_

To fill the series we have to divide the series→ aa_ / a_b / _ab / aa_
Let's check each option –
First option: abab; aa a / a b b / a ab / aa b (No repeated pattern has been found.)
Second option: baab; aa b / a a b / a ab / aa b (aab is repeated in the series.)
Third option: baba; aa b / a a b / b ab / aa a (No repeated pattern has been found.)
Fourth option: abba; aa a / a b b / b ab / aa a (No repeated pattern has been found.)

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