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

Let's analyze the pattern by counting the total number of spaces and letters. There are 12 positions in total. Let's try dividing the series into groups of 4:
_ U K _ | D _ K Z | D _ K Z

By observing the second and third groups, they are both D _ K Z. If we fill the second group's gap to match the sequence, let's see if a repeating 4-letter pattern emerges.
Comparing _ U K _ with D _ K Z:

1st letter is D

2nd letter is U

3rd letter is K

4th letter is Z

This gives a repeating unit of D U K Z.
Let's fill the pattern:

1st group: D U K Z

2nd group: D U K Z

3rd group: D U K Z

The inserted letters in sequence are: D, Z, U, U.