Insert the letters to maintain the cyclic order among r, s, t, u. Series: _ tu _ rt _ s _ _ usrtu _

Introduction / Context:
The letters r, s, t, u tend to appear in cyclical subsequences. Your task is to fill seven blanks (from a seven-letter option) to sustain that cycle without introducing impossible adjacencies.

Given Data / Assumptions:

• Schematic: _ tu _ rt _ s _ _ usrtu _
• We must preserve the ending block “usrtu” and ensure the cycle before it transitions smoothly into it.

Concept / Approach:
Favor fills that create r→s→u→r→t type rotations and avoid doubled letters that do not appear elsewhere in the skeleton. The concluding “usrtu” acts as a strong anchor.

Step-by-Step Solution:
Insert "rsurts": r tu s rt u s r s usrtu s. The lead-in to “usrtu” becomes consistent and no duplicate contradictions appear at boundaries.

Verification / Alternative check:
Test other options; they introduce double r merges or displace the expected order right before “usrtu”.

Why Other Options Are Wrong:
rtusru / rsutrr / rsurtr: Each creates at least one awkward adjacency or breaks the smooth link into “usrtu”.

Common Pitfalls:
Not aligning the pre-anchor segment with the fixed “usrtu” tail.

Final Answer:
rsurts