Choose the 4-letter fill that yields a consistent st–tt–ts tapestry: — s t t — t t — t t s —

Introduction / Context:
We are asked to restore a pattern built from t and s. The shown fragments suggest alternating clusters “st”, “tt”, “ts”.

Given Data / Assumptions:
Four letters fill four gaps in order.

Concept / Approach:
Test each candidate and confirm that every two-letter window cycles through the indicated clusters without contradiction.

Step-by-Step Solution:

Verification / Alternative check:
Write out the completed string and check successive bigrams.

Why Other Options Are Wrong:
They create doubled bigrams where the pattern calls for alternation.

Common Pitfalls:
Reading the blanks out of order.

Final Answer:
tsts