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

tf_nr_t_k_ru_fkn_utfk_ru

kufntrn

After dividing the series into 4 groups of 6 each and comparing the corresponding term. Repetitive unit is found to be tfknru

tfknru / tfknru / tfknru / tfknru

So, the code will be kufntrn