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.

c_cc_ac ca_cb_c cac_ba_

abcacc

After dividing the series in 3 parts of 7 letter each we get c_cc_ac / ca_cb_c / cac_ba_. On comparing different positions we get caccbac as a repetitive term.

caccbac / caccbac / caccbac

So the code required for the series is abcacc.