Correct Answer: baaa

Solution : Given:
ab_aabb_ _bb_

To fill the series we have to divide the series→ ab_a / abb_ / _bb_
Let's check each option –
First option: abbb; ab a a / abb b / b bb b (No repeated pattern has been found.)
Second option: abab; ab a a / abb b / a bb b (No repeated pattern has been found.)
Third option: baba; ab b a / abb a / b bb a (No repeated pattern has been found.)
Fourth option: baaa; ab b a / abb a / a bb a (abba is repeated in the series.)

So, the series becomes→abbaabbaabba. Hence, the fourth option is correct.