Select the combination of letters that when sequentially placed in the gaps of the given letter series will complete the series.

d_cd_c_ac_bcd_cd_c

abddab 

After dividing in three groups of six each we get d_cd_c / _ac_bc / d_cd_c. After comparing different positions we get as a pattern.

dacdbc / dacdbc / dacdbc

So the group having letters abddab will be used.