Fill in the blanks in the letter series “s_r_t_s_r” by choosing the set of letters that restores a structured repeating pattern.

Introduction / Context:
This problem provides a partially hidden letter sequence with several blanks. Your task is to select the option that, when inserted, creates a clear pattern using the letters s, t, and r. Many such questions are built around repeating or mirrored permutations of a small set of letters.

Given Data / Assumptions:

• Base pattern: s_r_t_s_r • Options: trts, rtst, trst, tsss. • Letters from each option are placed into the blanks from left to right. • The completed string should show a neat, structured pattern, often in equal sized blocks.

Concept / Approach:
Instead of guessing, we fill the blanks with each option and then inspect the full sequence. We look for repeated subpatterns, especially of length three, since there are three distinct letters involved. A strong candidate creates a sequence that can be split into equal sized repeated segments or symmetric permutations of the same letters.

Step-by-Step Solution:
Step 1: Substitute option (c) “trst”. Filling the blanks yields: strrtsstr. Step 2: Divide the completed series into groups of three letters. We get: str | rts | str. Step 3: Recognize the pattern. The first group “str” contains the letters s, t, r in natural ascending order by position in the alphabet. The second group “rts” is a permutation of the same letters, providing a rotated middle segment. The third group repeats the original “str”, giving a symmetric structure: str, rts, str. Step 4: This forward middle forward configuration is a clean and deliberate pattern typical of exam questions.

Verification / Alternative check:
Try replacing the blanks with other options. They create sequences such as strrttssr or srrttsstr which do not divide naturally into symmetric or repeated three letter blocks. Option (c) alone provides a visually and logically satisfying sequence that starts and ends with the same triplet and uses a permutation in the center.

Why Other Options Are Wrong:
• “trts”: Produces a string where the middle and end portions do not reflect a clear repetition of any one triplet. • “rtst”: Leads to an arrangement that cannot be split into three neat groups containing all three letters s, t, and r in a structured way. • “tsss”: Generates large clusters of s that break the balanced use of all three letters and do not hint at any intended symmetry.

Common Pitfalls:
Learners often look only at local adjacency, trying to avoid repeated letters, and miss broader structural patterns. Another mistake is to stop at the first option that seems somewhat regular without checking if it gives a strongly symmetric or repeated structure. Exam designers tend to embed elegant patterns that become obvious once the correct option is applied.

Final Answer:
The letter set that correctly completes the series is trst.