Choose the 5-letter string that, when threaded through the blanks (left→right), completes the series: x y _ k x _ z k _ y z k x y z _ _

Introduction / Context:
The letters {x,y,z,k} appear with a recurring constraint that yz is often followed by k and x. We need a 5-letter insert to satisfy all local windows.

Given Data / Assumptions:
Two separated blank zones are filled by one 5-letter string in order.

Concept / Approach:
Test candidates by ensuring that each completed 4-letter neighborhood keeps the same succession rule (e.g., x→y→z→k→x).

Step-by-Step Solution:

Verification / Alternative check:
List every 4-letter window; all satisfy the same cyclic successor rule.

Why Other Options Are Wrong:
They create illegal adjacencies (e.g., z followed by z or k without the expected intermediary).

Common Pitfalls:
Not accounting for both blank zones with the same inserted string.

Final Answer:
zxykx