ab _ aa _ bbb _ aaa _ bbba

Question: ab _ aa _ bbb _ aaa _ bbba
[A].

abba

[B].

baab

[C].

aaab

[D].

abab

Answer: Option B

Explanation:

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