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.

__A'B__B' AA'__BB'__A
A. A'BAB'
B. A'B'AB
C. ABAB
D. AB'A'B

Answer: Option C

Explanation:

Pattern of the series as shown below

AA'B/BB'A/A'AB/B'BA => ABAB

Hence, the missing letters are ABAB.