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

c_d_a_cbd_accbd_ac

bdcdd

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

cbddac / cbddac / cbddac

So the group having letters bdcdd will be used.