Select the combination of letters that when sequentially placed in the blanks of the given series, will complete the series.
yz_zx_xyx_z_zxzx

Let's count the total number of characters, which is 18. Let's try dividing the series into repeated blocks of 3, 6, or specific structures.
Let's check Option C: xyzzxy
Substituting the letters in order into the blanks:
1st blank → x
2nd blank → y
3rd blank → z
4th blank → z
5th blank → x
6th blank → y

The complete series becomes:
x y z y z x z x y x z z x z x z x y

Let's break this into groups of 3:
xyz | yzx | zxy | xzz | xzx | zxy → No obvious repeating pattern.

Let's look at a shifting pattern or block of 6 characters:
xyz zx_ ...
Let's analyze alternative patterns:
The series can be divided into repeating cycles of three letters x,y,z:
Let's test Option D: xyxzxy
Substituting the letters:
x y z y z x x x y x z z x z x z x y → No uniform pattern.

Let's test Option A: zxyxyy
z y z x z x y x y x x z y z x z x y

Let's reconsider a block of 3 elements rotating:
xyz, yzx, zxy
If we check:
1st block: x y z
2nd block: y z x
3rd block: z x y
4th block: x y z
5th block: x z x → doesn't fit standard block.

Let's test option C carefully with groupings:
If we fill with option C: x y z | y z x | z x y | x z z | x z x | z x y
Let's verify option D (xyxzxy):
xyzyzxxxyxzzxzxzxy

Let's test option B (xyzyyy):
x y z y z x z x y x y z y z x z x y
Let's split this into blocks of 3:
1st: x y z
2nd: y z x
3rd: z x y
4th: x y z
5th: y z x
6th: z x y

This creates a perfect cyclic pattern where the group of 3 shifts by one position each time:
(x y z) → (y z x) → (z x y) → (x y z) → (y z x) → (z x y)

The blanks are filled by: x, y, z, y, y, y.
This perfectly matches Option B.