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

Let's count the total number of characters including blanks, which is 24. We can divide the series into 4 repeating patterns of 6 letters each:
kb_fg_∣k_df_t∣kbd_gt∣kbdf_t
By comparing the blocks with each other, we can deduce the complete pattern is k b d f g t.

1st block: kbdfgt → filled letters: d, t

2nd block: kbdfgt → filled letters: b, g

3rd block: kbdfgt → filled letters: f

4th block: kbdfgt → filled letters: g

The sequentially filled letters are: d, t, b, g, f, g.