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.

df_h_rk_ft_gr_d_th_rk

tgdhkfg

After dividing the series in 3 parts of 7 letter each we get df_h_rk / _ft_gr_ / d_th_rk. On comparing different positions we get dfthgrk as a repetitive term.

dfthgrk / dfthgrk / dfthgrk

So the code required for the series is tgdhkfg.