Which one set of letters when sequentially placed at the gaps in the given letter series shall complete it ?

ab _ aa _ bbb _ aaa _ bbba

A. abba

B. baab

C. aaab

D. abab

Answer: Option B

Solution(By Examveda Team)

The series is abb/aaabbb/aaaabbbb/a. Thus the letters are repeated twice, then thrice, then four times and so on.