Correct Answer: abab

Solution : Given:

a_ _b abba_abb a_ba

Let's check the given options –

First option: abab
The series becomes a ab babba a abba b ba.

The pattern in the series becomes a a / b b / ab / ba / a a / bb / a b / ba.
Second option: abba
The series becomes aa b babba b abba a ba.

There is no pattern in the above series.

Third option: aabb
The series becomes a aa babba b abba b ba.
There is no pattern in the above series.

Fourth option: aaab
The series becomes a aa babba a abba b ba.
There is no pattern in the above series.

Hence, the first option is correct.