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

c_ _ca_ _ecb_ _adce_ _dcad_e

bddcdcbc

After dividing the series into 3 groups of 8 each and comparing the corresponding term. Repetitive unit is found to be cbdcadce

cbdcadce / cbdcadce / cbdcadce

The code to be used is, bddcdcbc