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.

dc_dba_ca_bad_ad_a

addcb

After dividing the series in 3 parts of 6 letter each we get dc_dba / _ca_ba / d_ad_a. On comparing different positions we get dcadba as a repetitive term.

dcadba / dcadba / dcadba

So the code required for the series is addcb.