1. 

In the following question, a letter series is given. Some of the letters of the series are missing. This missing letters are given in same order as one of the four/five alternatives below the series. Find out the correct alternative.

aa__aabb__b__aa__aabb__bb
A. bbbaa
B. bbbba
C. aabbb
D. babba

Answer: Option D

Explanation:

Pattern of the series as shown below

aabaa/bbabb/aabaa/bbabb => babba

The sequence is made of two patterns 'aabaa' and 'bbabb'

Hence, the missing letters are babba.