Choose the 4-letter string that completes the pattern: _stt_tt_tts_.

Introduction / Context:
We must insert one ordered 4-letter option into the blanks to yield a consistent s/t pattern that repeats short motifs. The string features clusters “stt” and “tts” that hint at overlapping triplets.

Given Data / Assumptions:

• Skeleton: _ s t t _ t t _ t t s _
• Options: tsst, sstt, ttst, tsts (letters used in order).

Concept / Approach:
Look for a fill that supports rolling triplets like “s t t” and “t t s” without breaking adjacency when overlaps occur. The candidate should produce a smooth alternation of single s’s introducing or trailing blocks of t’s.

Step-by-Step Solution:
1) Try “s s t t”. Place s at blank1: s s t t s… aligns two leading s’s into a natural preface before a “tt”.2) Continue: inserting s at blank2 preserves “…stt s tt…”.3) Insert t at blank3 to maintain “…tt t tt s…”.4) Insert t at blank4 to complete the tail “…tts t”, leaving well-formed “stt/tts” overlaps.

Verification / Alternative check:
Read the finished line with sliding windows of length 3 and note the recurrence of “stt” and its rotation “tts”.

Why Other Options Are Wrong:

• tsst / ttst / tsts: Each creates at least one local inversion (e.g., “tst”) that breaks the intended triplet overlaps in the mid-segment.

Common Pitfalls:
Matching only the first blank; always confirm all four placements.

Final Answer:
sstt