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.

d_cc_ab_ac_dabd_ccd_b

addcaa

After dividing the series in 3 parts of 7 letter each we get d_cc_ab / _ac_dab / d_ccd_b. On comparing different positions we get daccdab as a repetitive term.

daccdab / daccdab / daccdab

So the code required for the series is addcaa.