The correct answer is (b) aab.
The series is made up of two alternating sequences: aba and ba. The first sequence starts with “a” and ends with “b”, and the second sequence starts with “b” and ends with “a”. The third and fourth letters of each sequence are the same.
In the first sequence, the third letter is “b” and the fourth letter is “a”. In the second sequence, the third letter is “a” and the fourth letter is “b”. Therefore, the third and fourth letters of the missing word must be “a” and “b”, respectively. The only option that satisfies this condition is (b) aab.
Option (a) aaa is incorrect because the third and fourth letters are both “a”. Option (c) bab is incorrect because the third letter is “b” and the fourth letter is “a”. Option (d) baa is incorrect because the third letter is “a” and the fourth letter is “a”.