A letter series is given below in which some letters are missing. Select the option that gives the letters that can fill these blanks in that order.

ab_a_bab_ad_abc_db

cdcba

After dividing the series in 3 parts of 6 letter each we get ab_a_b / ab_ad_ / abc_db. On comparing different positions we get abcadb as a repetitive term.

abcadb / abcadb / abcadb

So the code required for the series is cdcba.